[Dovecot] authentification failure: double quotes in password

we use Passdb LDAP with authentication binds against an Active Directory Server. The problem with one of our users seems to be the usage of double quotes in his password, as the windows-login works:

/var/log/dovecot-info.log

auth(default): Info: ldap(testuser 77.22.xx.xxx): invalid credentials (given password: xxxxx"xxxxxxx)

Which chars in passwords does the dovecot auth process not like? Is there something similar to auth_username_chars for passwords?

thanks, martin