It appears the MTA is closing the connection right after you send the "MAIL FROM:" string.
Net::SMTP=GLOB(0x20461c8)>>> MAIL FROM:<sender address shown here>
Net::SMTP: Net::Cmd::getline(): unexpected EOF on command channel: at
+ /srv/www1-ssl/cgi-lib/sendmail.pm line 76.
So, it's either the MTA itself, or some external thing like AppArmor (enabled by default in the OpenSuse version you mention).
I would look at the logs for whatever your MTA is (exim maybe?) as well as AppArmor, SELinux, or whatever other external security stuff you may have running.
Maybe also try "service boot.apparmor stop", and see if turning off AppArmor makes a difference. Not the right solution, but quick to try.