Well the log files are clear as hell.^^
Somehow you managed to change the databased from postfix.
Try: /var/www/imscp/engine/setup/imscp-reconfigure -r mta for it.
Maybe that can fix the postfix issues.
To the apache part, same too.
The certificate is missing. Means it got deleted or changed. How did you installed the certificate?
That path name is what bothers me, normally there shouldnt be a certificate. I guess you tried to manually change the files.
You should either use the letsencrypt plugin or check certbot out.