Evaluating further I'm now almost sure, that this is not a proftpd problem.
On my local machine I also get the start-message
warning: "ProFTPD" address/port (192.168.0.123:21) already in use by "Debian"
but in ISPconfig the Service is online and connection with WebFTP is possible.
When I run proftpd in debug mode (proftpd -nd6) and connect through WebFTP, the local server gives me a bunch of messages of this type:
athena.myhost.tld (localhost.localdomain[127.0.0.1]) - dispatching auth request "getpwnam" to module ...
On the problem server I get not a single message, when I try to connect with WebFTP. The proftpd server is simply not reached, even though he's listening on his IP and on localhost