Login to server with account / password fails suddenly

I have various mail accounts set up in my Thunderbird. All of them used to work just fine until yesterday. Since yesterday I receive an error for one of them that "Login to server ... with user ... fails." I have a second account with the same mail provider which works just fine with the same configuration settings. The account in question used to work just fine for several years without any changes. I tried saving the account data, deleted it from Thunderbird and set it up again - no success. Same error message. Connecting to the mail account via browser with the same credentials works alright. At the moment I'm at a complete loss what could be the problem - anyone having some brilliant idea?