The problem is apparently not related to the characters in the foldername-
I had turned some logging options off then on again in Dovecot at the same time but did not suspect this to be the problem.
I don't know yet which of the two it was, maybe both, but the options I had turned off and on were:
auth_verbose = yes
auth_debug = yes
I had disabled both options by using a # in front.
Perhaps there is a problem in dovecot-auth, backport v.1.2.10 ?