Thank you for your advice once more. I must say that once I stopped xinetd and started dovecot, I had successful access to my webmail. And the mail server was working fine for everyone else. So then I realized that for my username the webspace was only 50MB which is fine for all other accounts, but since I am the administrator the website takes up quite a bit of space exceeding the limit.
Solution: all I did was login to ISPConfig as administrator or reseller or even client of the webspace, select users/emails tab. Select my user account and change the 50MB webspace limit/quota to -1. Thank you Falko, love your program, and I will upgrade to the new version if possible this year..