The cause is probably a corrupted /opt/psa/admin/conf/httpsd.pem file. Plesk create automatic backups of that file, so you should be able to restore it from such a backup:
ls -l /opt/psa/admin/conf/httpsd.pem*
should result in something like
-r-------- 1 root root 1922 Jan 27 12:59 /opt/psa/admin/conf/httpsd.pem
-r-------- 1 root root 6525 Jan 27 12:55 /opt/psa/admin/conf/httpsd.pem.sav
Then you can mv /opt/psa/admin/conf/httpsd.pem /opt/psa/admin/conf/httpsd.pem_not_working cp -p /opt/psa/admin/conf/httpsd.pem.sav /opt/psa/admin/conf/httpsd.pem