You say this so confidently but none of those solutions work, and contacting support they just suggest those same fixes and then say to look at the forums without being able to actually fix anything. Truly hopeless. They say that a fix should come soon but it's a years old bug in a game that isn't actively developed.
I don't even care about the fact that I need to re-level my characters but the bug causes you to be permanently locked out of earning milestone cosmetics as the milestones are already marked complete with the respective cosmetics still locked.