Haven’t got an idea, but just saying that I did a quick search on gkr-pam issues and couldn’t find anything directly tied to dovecot. A number of issues over the years have been posted about login authentication related to the Display Manager but it’s hard for me to apply any of that to a mail server system unless you can verify your gkr-pam error is thrown in other, non-mail mail situations.
Or, I guess perhaps if your mail server is set up to serve regular system accounts as mail accounts (IIRC the YaST mail server setup does this), which I find inconsistent with mail systems I’ve managed then those posted issues might be relevant.
In any case, for practically all gkr-pam issues which have been posted in the past have been resolved by upstream debugging, not by the end User. If your system worked before and this problem suddenly appeared, I’d recommend first posting and extremely detailed description of your setup, enough that the problem can be replicated and if no one can fix it then submit a bug to the bugzilla.