Thanks nuxwin - the first point were also my thoughts, but I was not shure...
How can I backport the pm - I'll search first to solve it alone, if I do not succeed, I'll come back 
Greetings
hempel
mhm - I don't think that it has to do with subdomain.pm - because the deletion of an alias-domain does not work.
Now I've deactivatet adminer, the first failure message disappears, if I try to start imscp-rqst-mngr manually.
But the second message with apache_pohp_fpm.pm is still relevant and the alias won't be deleted.
Now I've manually deleted the entry in the database (imscp - table domain_aliasses entry of the spezific alias) - and now the requestmanager starts manually witout any complaint. But the folder(s) of that alias were not deletet.
Because I've to delete some other users / domains / aliases on that server it would be nice, if I could do that with imscp - how can the issue be fixed?
somtime later:
Now I've unmounted the log-dir of a domainalias which should be deleted, and did delete in imscp. But noting in the database nor the directory-structure of the domain will occur - the status is "wird gelöscht..."
When I manual start the request-manager the samt failure message appears:
[FATAL] iMSCP::Debug::__ANON__: Could not run mount command: Unknown error at /var/www/imscp/engine/PerlLib/Servers/httpd/apache_php_fpm.pm line 1410.
Now I give it up an hope, that someone knows a solution....
it will not let me in peace 
Now I did look into the perl-module, which gives the failure. The line 1410 is in sub umountLogsFolder, in the part to mount folder(s)... there must be someone a problem to mount the given folders
No, I can't understand whats going on, sorry - I need further help 
sometime later:
I tried to delete a customer / whole domain - the same as above, nothing runs.
Now I need urgent help...