As far as I'm aware, a lot of the trash-talking the Middle Ages gets is as a result of the Renaissance.

First of all, we have to realize that the Renaissance view of the Middle Ages is obviously skewed. They just got subjected to a population crisis, an economic crisis, and the worst plague in recorded history causing even more of the aforementioned. So, the Renaissance view of the Middle Ages didn't start off very well, since the Middle Ages they were familiar with was one that wasn't a great time to live. The High Middle Ages were markedly better, but people back then weren't very into writing the historical record, so a lot of how reasonable things were got lost in the intervening period of crisis.

Secondly, consider that the Renaissance period was, like, way into the Classical period, particularly in distinction to the Middle Ages. So, everything that the Classical period did well, they would often claim the Middle Ages sucked at, and everything the Middle Ages (as they knew them from the last century or so) sucked at, they would often claim the Classical period did well. They got these things wrong an awful lot, and had a lot of bias towards the Classical period and against the Middle Ages.

Thirdly, consider that while the Middle Ages certainly did have a fairly robust system of education (eventually, anyway) with the founding of the first universities, the Renaissance - as it often did - tended to reject foundations provided by the Middle Ages and embraced Classical methods instead. This led to the re-adjustment of the education system into the one that we know today: the seven liberal arts were popular in medieval education, but the humanists added several subjects, including the academic study of history.

So, given the bias of Renaissance scholars against the Middle Ages, and given that they added history as an official subject to their academies, it makes sense that modern history - informed by prior historical efforts - would inherit the anti-medieval bias of the Renaissance. I would suggest that this is the origin of many of the myths you refer to, and they probably live on in popular history because they have long since entered the public conscious, whereas academic efforts to correct them have remained fairly niche.

To answer your particular questions -

Is it the result of 'just bad research'?

Probably sometimes.

Or perhaps some sort of secular Enlightenment anti-religious reaction?

Probably other times.

Maybe just a really unfortunate series of wrong or half-right conclusions by various, unrelated historians over the course of several centuries?

Likely a lot of this, too.

Or quite the opposite, an actually concerted, thought out effort of a select few individuals at some sort of historical revisionism?

Sometimes this, though I'd imagine it's more rare than honest mistakes plus societal bias.

In other words, it's probably complicated. A set of myths this pervasive rarely builds up quickly, and we know that people were hating on the Middle Ages six or seven centuries ago, if not earlier. Just speaking statistically, it's likely that all the things you mentioned - and more - took place at some point in those hundreds of years.

Sorry I can't provide a more powerful answer than "yeah, the Renaissance kind of screwed it up for everyone and momentum carried it through to today," but that's basically the extent of my knowledge.

Also sorry I don't have any real sources - this all comes from lectures I've attended, so I'm basically taking my prof's word for it for a lot of this, and that's often fine in solving idle curiosity, but it's not the sort of rigor people around here rightfully expect. Hopefully somebody can come along and verify or debunk it and provide better sources as appropriate, but until then hopefully this reply tides you over.

There's something of a trickle-down effect in history, where new ideas take a long time to enter popular consciousness. As a consequence, while a lot of old ideas about the middle ages that were once accepted by scholars are now thoroughly discredited in academia, this academic consensus hasn't always spread to pop culture. Some of the concepts you mention in your question (especially the 'Dark Ages' and feudalism) were once accepted by academics until a few decades ago, and have only recently been discredited. Pop culture hasn't caught up yet.

Feudalism is slow to die, I think, because it's a debate about legal history which is really hard to communicate to a popular audience. Academics have criticized the concept of feudalism since the 60s and 70s (but still taught it, often because it was such an easy way to teach 100s of years of history to undergrads in a few straightforward lectures), but the real death-knell was only sounded 20 years ago with Susan Reynold's Fiefs and Vassels (1994). Since then, while there's been lively debate among medievalists about how we should talk about the middle ages, this debate hasn't had time to enter the popular consciousness. This is partly because medieval history is a relatively small field, and it takes time to update all the textbooks on the market (some of which haven't been reissued in quite some time, as there's not a huge demand for them like there is for, say, intros to American history). But it's also because the new models that have replaced feudalism are very messy, relying on discussions of legal history and complex political structures that can't be summed up into a quick sound bite. It's hard to replace an easy-to-grasp model that's become a part of popular discourse ('that 3rd world country is practically feudal!') when the alternative starts with 'well, things were messy and changed a lot through time...' So popular consciousness is slow to catch up with academic debates about feudalism, and academics have perhaps not done as much as we could to get the new message out there (though the tide is steadily turning), hindered further the unsexiness of the new model.

The 'dark ages' was, like /u/Reddit4Play says, an idea originally pushed by the Renaissance. But it was still widely accepted through at least the 80s, if not longer. One of my committee members, when starting graduate school in the late 80s, was told not to study 'the dark age' because 'there wasn't anything you could say about it' since there were so few surviving texts. It turns out this wasn't true, but we can only say that now because early medieval historians developed new and exciting ways to read the few sources we have, starting in the 80s and 90s, and because archaeology has EXPLODED the amount of evidence we have available. But these are recent developments, and - as with feudalism - have taken a long time to spread to popular consciousness. Unlike feudalism, getting people to give up the idea of the 'dark ages' has been easier, because it's easier to show someone material evidence that life wasn't as bad as we used to think in the early middle ages (and, for that matter, to prove that we can know a lot about the period - it isn't 'dark' to our understanding) than it is to convince someone that debates in the finer points of legal history (feudalism) matter.

So the field has advanced, but these advances haven't always succeeded in entering popular consciousness. This is partly because academics have a bad tendency (reinforced by institutional pressures) to talk to each other instead of to a wider audience. Publishing a popular-press book isn't rewarded as highly (in terms of promotions and institutional reviews - ie not getting fired) as publishing in an academic press, even if that means your books end up being more esoteric and hundreds of dollars more expensive - that is, out of reach of your typical casual student of history. It's also not helped by the fact that the old model, with its fur-clad barbarians, burning Roman ruins, religious fanatics burning witches, and wedding-night-ruining prima nocta rapists, makes good television and is, more importantly, what people want when they watch a 'medieval' movie or documentary. One of my professors once gave an hour-long interview with the History Channel about how civilized barbarian people in the early middle ages actually were; they only aired about 10 seconds, and made it sound like she was criticizing barbarians for having terrible hygiene.

So the misconceptions are partly the natural lag that happens between specialists and laymen - it takes time for new academic knowledge to become popular knowledge. They're partly from institutional barriers - slow textbook revisions, too few courses on medieval history being offered in universities beyond the 100-year survey classes, and institutional disincentives to publish cutting edge research in easily accessible formats. And they're partly a product of wider popular disinterest in giving up theromantic, fur-clad image of the middle ages as Europe's barbarian hey-day (to the point of frequently just ignoring historians to tell a 'better' - ie, less accurate - story).

If those are myths, than what was reality? Did the (RC) church not have a major role, was a lot of what happened in fact written down, was there no feudalism or was feudalism all good?

I would highly recommend

1. Marcus Bull. Thinking Medieval: An Introduction to the Study of the Middle Ages (New York: Palgrave Macmillan, 2005).

and

2. Norman Cantor. Inventing the Middle Ages (1993).

They address your questions directly and are fun reads. Here are a couple of relevant citations from Bull:

"We are all complicit in the creation of the pop-cultural Middle Ages by being consumers" (p.39)

"Perhaps one of the biggest challenges faced by anyone studying the Middle Ages is to unthink a raft of modern assumptions and values about the morality of violence, because only then is it possible to understand how people with entirely different approaches were able to function" (p.131)

"In a way, therefore, the sort of rubbishy medieval history peddled by pseudo-historians is one manifestation of popular culture’s insistence on simplifying the Middle Ages as far as possible...Alterity is about the tremendous variety within and between medieval cultures." (p.141)

Hope this helps a little. Happy reading!