the english version of the message is "Domain panel1.<tld> is unavailable." (replaced the actual domain by <tld>)
I did the following steps:
- log in as reseller
- switch to customer view
- add customer
- insert panel-domain in field "Domain name"
- URL forwarding to "yes"
- insert panel-domain with port 8443 and protocol https in "Forward to URL"
- select "PROXY" and "PRESERVE" in "Forward type"
- click "Next Step"
- error occurs