The best examples I can think of are the blood libel and accusations of well poisoning, both of which were attributed to Jews.

The medieval blood libel began in Norwich, England when a twelve-year-old boy named William was found murdered in the woods and his family blamed the Jews for it. It didn't take on conspiracy tones until around 1150 when a monk named Thomas of Monmouth wrote a hagiography of William that claimed that he had been crucified by Norwich's Jews as part of a pan-Jewish plot to regain Jerusalem. Thomas' informant was Theobald of Cambridge, who claimed that every year, an international council of Jews chose a country to sacrifice a Gentile child and that year, the Jews of Norwich had been selected. From there, accusations of Jews sacrificing Christian children also occurred in France, Germany, and Italy.

The well poisoning conspiracy theory arose during the Black Death, when it was claimed that Jews were causing the plague by, well, poisoning wells. It began in 1348 when a Jew named Agimet of Geneva was tortured until he confessed that his Rabbi gave him poison to poison the wells in Venice and Toulouse, among other places. From there, Jews in Germany, Italy, Spain, and other parts of Switzerland were tortured into confessing that they'd poisoned wells to spread the plague.

Sources:
Joseph P. Byrne, The Encyclopedia of the Black Death
Alan Dunes, The Blood Libel Legend: A Casebook in Anti-Semitic Folklore
John Kelly, The Great Mortality: An Intimate History of the Black Death
Darren O'Brien, The Pinnacle of Hatred: the Blood Libel and the Jews
E. M. Rose, The Murder of William of Norwich: The Origins of the Blood Libel in Medieval Europe
Philip Ziegler, The Black Death