The password for the incoming gmx server will be the same as the one for the gmx smtp server.

I can no longer receive emails. The password is requested, but this does not work; it is the same as for the SMTP server. The IMAP server is set up (server settings), see picture. It does not appear under the access data (Niels.Wichmann@gmx.de), see image. The SMTP is in the list. The email account works on my smartphone. How can the problem be solved?

The IMAP settings are correct, so I suggest resetting the password on the gmx website, removing the smtp:// entry for the account in Saved Passwords, restarting TB, and entering the new password when asked. You will have to enter the new password on the phone as well.

I have changed the password to GMX and deleted the smtp:\\. Now when I open Thunderbird I can enter the new password and retrieve, read, send, etc. the emails. I have to enter the password every time I open TB and neither the IMAP nor the SMTP account is displayed in Saved Passwords.

When you enter a password, there should be a checkbox to store it in Saved Passwords. Do you have an external password manager that's part of some security apps? That might be intercepting the password process in TB.