This is the user DN:
cn=Klara Fall,ou=People,dc=[domainname],dc=de
According to your Dovecot configuration
auth_bind_userdn = cn=%u,ou=People,dc=**[domainname]**,dc=de
if you login with "klarafall" it will be expanded into
cn=klarafall,ou=People,dc=[domainname],dc=de
which is not the correct DN for Mrs Klara.
So if you login with "Klara Fall" it should work, but that will probably mess up the things on Dovecot filesystem.
I am strongly against setting a static DN when dealing with LDAP authentication. LDAP servers are optimized to serve search requests, so let yours do the job. Allow Dovecot to lookup the correct DN based on the attribute you supply (uid) and then authenticate.
This should be achieved if you comment out the auth_bind_userdn line.
Paolo Cravero