askvortsov literally deleted that index, after that opened flarum and clicked button to "update", that brought me to index page. While i tried to login, i noticed my username does not exists and forum name was null. So i just inserted new user, generated bcrypt password and inserted user group connection for admin.
After that i could login and enter admin panel, but im pretty sure panel is missing a few things (like i said, monkey business)
EDIT: Pretty sure these line causes that error, that is one out of two places that mentions last_posted_at in migrations, and only one that actually touches migrations. So i would start from there by simply dropping that index before trying to rename it.
