Hello chort1,
From your error message it seems that your user database might just be lacking the __DEFAULT__ user which is very strange because it is always supposed to be present. You could try going to your Access Server's console and logging in with root privileges, then going to /usr/local/openvpn_as/scripts/ directory, and running the command ./sacli userpropget, and see if __DEFAULT__ user is in there at all. If it's not, you might want to reach out to our support ticket system and ask how to restore the __DEFAULT__ user. That would at least resolve the error message and allow for things to likely proceed as they should.
The actual configuration parameter for the multiple session per user setting in the configuration database is "vpn.server.duplicate_cn". You can set it manually on the command line as root user with;
/usr/local/openvpn_as/scripts/sacli --key "vpn.server.duplicate_cn" --value "true" ConfigPut
/usr/local/openvpn_as/scripts/sacli start
You can use false to turn it off, true to turn it on. And if you remove it, it will go to its default which I believe is true;
/usr/local/openvpn_as/scripts/sacli --key "vpn.server.duplicate_cn" ConfigDel
/usr/local/openvpn_as/scripts/sacli start
For documentation see here;
https://openvpn.net/vpn-server-resources/#documentation-subtab-command-line
Kind regards,
Johan