The errors are replicated in the miniserv.error log:
[18/Aug/2016:08:34:26 +0200] [105.233.110.14] /session_login.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:08:34:47 +0200] [105.233.110.14] / : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:08:34:48 +0200] [105.233.110.14] /mailbox/ : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:08:34:56 +0200] [105.233.110.14] /mailbox/index.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:08:34:59 +0200] [105.233.110.14] /mailbox/view_mail.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:08:39:45 +0200] [105.233.110.14] /mailbox/index.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:08:40:16 +0200] [105.233.110.14] /mailbox/view_mail.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:09:18:37 +0200] [105.233.110.14] /mailbox/reply_mail.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:09:18:54 +0200] [105.233.110.14] /mailbox/index.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:09:18:56 +0200] [105.233.110.14] /mailbox/view_mail.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:09:19:03 +0200] [105.233.110.14] /mailbox/index.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:09:19:07 +0200] [105.233.110.14] /mailbox/index.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.
[18/Aug/2016:09:19:09 +0200] [105.233.110.14] /mailbox/reply_mail.cgi : Perl execution failed : Undefined subroutine &WebminCore::embed_scripts called at /usr/libexec/usermin/authentic-theme/authentic.pm line 223.