usermin loads forever > error message

currently I am migrating one server to a new, fresh installed system running

Debian Linux 9 Usermin 1.741 webmin 1.892 Virtualmin version 6.03

Usermin loads and loads and loads for like half a minute after pressing login button then the left menu appears and it continues to load and load and load for another half minuten. Then - instead of the mailbox content - a page with 'Error' is displayed > see attachment

the specific /var/usermin/miniserv.error looks like:

Use of uninitialized value $mailbox::config{"server_name"} in string eq at /usr/share/usermin/mailbox/mailbox-lib.pl line 1323.
Use of uninitialized value $mailbox::config{"server_name"} in string eq at /usr/share/usermin/mailbox/mailbox-lib.pl line 1337.
Use of uninitialized value $mailbox::config{"server_name"} in string eq at /usr/share/usermin/mailbox/mailbox-lib.pl line 1337.
Use of uninitialized value in numeric eq (==) at /usr/share/usermin/mailbox/mailbox-lib.pl line 2234.
Use of uninitialized value $file in substitution (s///) at /usr/share/usermin/mailbox/mailbox-lib.pl line 667.
Use of uninitialized value $file in substitution (s///) at /usr/share/usermin/mailbox/mailbox-lib.pl line 668.
Use of uninitialized value $file in substitution (s///) at /usr/share/usermin/mailbox/mailbox-lib.pl line 670.
Use of uninitialized value $file in substitution (s///) at ./mailbox-lib.pl line 667.
Use of uninitialized value $file in substitution (s///) at ./mailbox-lib.pl line 668.
Use of uninitialized value $file in substitution (s///) at ./mailbox-lib.pl line 670.
Use of uninitialized value $mailbox::in{"jump"} in pattern match (m//) at /usr/share/usermin/mailbox/index.cgi line 76.
Use of uninitialized value $mailbox::in{"start"} in int at /usr/share/usermin/mailbox/index.cgi line 84.
Use of uninitialized value $mailbox::in{"start"} in int at /usr/share/usermin/mailbox/index.cgi line 84.
Error:
Status: 
Closed (fixed)

Comments

added excerpts of the files causing errors

not sure if its important but I accessed usermin not from the domain the specific users mailbox is running on but from the servers hostname adress > eg server.serverdomain.net

I also do this on an older server running debian 7 without any issues.

Status: Active ยป Closed (fixed)

error fixed. looks like dovecot could not find the certificate. dovecot: imap-login: Fatal: Couldn't parse private ssl_key: error:0906D06C:PEM routines:PEM_read_bio:no start line: Expecting: ANY PRIVATE KEY reassigend a certificate and everything is running smoothly.