Okay, seems to be a problem since 2008:
Openfire LDAP Quotation (encloseDNs) Problem and Fix.
It's a pity :-(
I tried to compile it with the workaround as described in the post, but it still doesn't work.
The interesting thing is that the admin credentials are not in quotes and working fine, but the userdn are enclosed excepting the base dn.
In addition I tried to set ldap.encloseuserdn to false, but it have no effect.