Wrong Microsoft Account connection, can't reset, so no multiplayer possible

When I first tried to connect my Microsoft Account to play online multiplayer, it connected to an unknown account that was not mine. I could not reverse that. I completely deleted minecraft and re-installed it but nothing helped so far.

When I try to connect my own account, it tells me that this is not possible because it is already connected to the wrong one. Because I have no access to that foreign account, I'm lost in this issue.

Please help!