I didnt change it, there are alot of other places in the database as Latin1_swedish_ci, how can I change all of them to "utf8_general_ci" ?They should all be "utf8_general_ci". You should change them. Check the other tables and fields as well.
Did you deliberately change those collations to "latin1_swedish_ci"?
FYI, you should be able to reproduce the error if you try to create a thread with the title indicated in the error: