Your professor is correct. What s/he's referring to is a diplomatic mess that later became known as the Sixtus Affair, in which Austria-Hungary tried to conclude a separate, secret peace with the Entente that didn't involve Germany.

Empress Zita of Austria-Hungary had brothers fighting on both sides of the war. One of her brothers in particular, Prince Sixtus of Bourbon-Parma, was serving as an officer in the Belgian military. At Zita's behest, he agreed to act as an intermediary between Kaiser Karl and the French, carrying secret correspondence between both parties in which they outlined their conditions for peace. The chief conditions on the part of the Entente were as follows:

• that Austria-Hungary recognize France's claim to Alsace-Lorraine (which had been seized by the Prussians during the German Unification)
• that Belgian sovereignty be restored
• that the Serbian Monarchy be recognized and that Serbia be given the parts of Albania that it currently occupied
• that Austria-Hungary make no attempts to block Russia's claim to Constantinople

Kaiser Karl agreed to all of these demands but the last. In particular, he supported France's claim to Alsace-Lorraine. A copy of this letter was kept by the French (which was a no-no: both parties in the negotiation were supposed to destroy the correspondence afterwards).

When the French Prime Minister Briand was replaced with Ribot, the negotiations began to stall. Ribot insisted that British Prime Minister David Lloyd George be informed, who in turn insisted that Italy be consulted (Italy, of course, wanted parts of the Austrian Tyrol). While Charles was willing to agree to this demand, Italy's establishing of a protectorate over Albania more-or-less invalidated the earlier terms set by the Entente and the whole thing floundered. But that wasn't the end of the affair.

After the treaty of Brest-Litovsk was signed, Austria-Hungary's foreign minister Ottokar Czernin gave an inflammatory speech in Vienna in which he stated that the main obstacle to peace in Europe was the French and their refusal to negotiate on Alsace-Lorraine. Prime Minister Clemenceau was stung by this false claim and began to reference the earlier peace talks. But Czernin wouldn't back down. While he conceded there had been peace talks, he continued to insist that it was French who had refused to budge, and presented a statement written by Charles denying Clemenceau's claims. The pro-Habsburg sources seem to indicate that the Kaiser wrote this statement under duress, recovering from a heart attack at the time and trying to stop Czernin from leaving the palace after the fact, but you should take that with a grain of salt.

At this point, the French press published Kaiser Karl's letter (the one in which he promised to support French claims to Alsace-Lorraine). This blindsided Austria-Hungary's German allies, destroying Karl's credibility and eliminating what little autonomy Austria had left as the junior power in that alliance.

I hope this at least gives you some leads: what I gave you was a fairly quick overview, and I confess that the sources I've cited are written from a pro-Habsburg slant. I suspect you can find some other sources that go into deeper detail.

Sources: "The Last Empress: Life and Times of Zita of Austria-Hungary, 1892-1989", "A Heart for Europe: the lives of Emperor Charles and Empress Zita of Austria-Hungary"