I have perhaps found an error.
tail /var/log/messages gave me:
Mar 26 10:54:36 whitespace saslauthd: ipc_init : listening on socket: /var/run/sasl2//mux
There is a double slash between sasl2 and mux. This must be an error. The question is: can it be THE error? Where can I find that path in the sasl config?