I can`t test this right now, as I exchanged ipopd-ssl and uw-imapd-ssl for dovecot. If I find the time I will reinstall ipopd-ssl and uw-imapd-ssl (approximately on Tuesday) and post the output.
With dovecot installed all the logins work well (webmail, mailuser, pop3 and pop3s).
But I tried ipopd-ssl and uw-imapd-ssl on another server (no ISPconfig) . I have the same problems there. Netstat output is quite long (as the server is running LDAP, samba, hylafax etc.). Non SSL pop3 calls are not working on this server, either.
Can I find a log for pop3 somewhere (except mail.log)?
Funny thing is that I can put nonsense (lines like bliblablub 0) into /etc/c-client.cf and do not find any error in any log. So it seems to me that /etc/c-client is not being read on my systems.