Thank you for your answer, after your comment I really started to think the problem was on my own side, and it was actually.

I connected to my account directly on the lemm.ee’s website, and I changed my password, just to try and see what happens, and it worked !

I tried to login in Mlem directly with your recommandation for the fields and the new password and it was successful first try

Sorry to have bothered you and thanks again for your help

Just tried right now, it doesn’t work either, I tried with “@” and without “@”, and I tried with the instance’s name included too

I also checked one more time my password and it’s correct since I’m able to connect on the web client