The server returned the error: Invalid credentials(Failure)

The server returned the error: Invalid credentials(Failure)

Hello,

I have issues with connecting to my mail account at koszek.com

I'm using Apple Mail and I haven't made any configuration changes recently.

However today my Apple Mail client returns:

The server returned the error: Invalid credentials(Failure)

Could someone help me diagnose this problem? I only use 0.28GB so space isn't the issue. Also I've cleaned many IMAP folders from the web interface.

W.