@Palandre,
Thomas Aigner is a German geologist (sedimentology), professor at the Center for Applied Geoscience of the University in Tübingen.
He was a member of the Academy of Sciences and Literature Mainz* in the class "Mathematics and Nature Sciences" for the discipline 'Geosciences'.
*The Academy of Sciences and Literature Mainz is a state institution of the state of Rhineland-Palatinate.
He said in his resigning letter that he had no medical knowledge [Aigner is well known for its unique collection of drill cores. Rocks, millions of years old. Shell limestone, clay slate and sandstone.], but resigned with reference to the "7th ad hoc statement" of the National Academy of Sciences Leopoldina of 8 December 2020.
His main argument for resigning from the Mainz Academy was that no member of this academy (and none of any other academy) had protested against the above-mentioned decision of the National Academy: "I have decided to take the certainly unusual step of resigning as an expression of my personal protest."
He doesn't say anything about a "hoax" or that the virus didn't exist.