One user cannot login, no matter what password

I am using Linuxserver.io’s docker image, I have had this running for years now.
A while back I am not able to login to the web client, the sync clients does still work.
When I enter the credentials, a spinner starts in the password field and after 10-15 secs the page blinks and the fields are empty again.
The admin account still works, so I tried to change the password, did not help.
So I created a new user, it worked and I can login with that.

So I copied the password from the first user and added it to the new user, now the new user cannot login.

What is happening here? Please help!

Please use occ a very powerfull tool.

https://docs.nextcloud.com/server/stable/admin_manual/configuration_server/occ_command.html#user-commands-label

Just in case you ask :wink: