You can enable system user from the command line, for that:
1) stop netxms server
2) run
nxdbmgr reset-system-account
3) login specifying user system and password netxms
4) enable admin back
5) disable system user
1) stop netxms server
2) run
nxdbmgr reset-system-account
3) login specifying user system and password netxms
4) enable admin back
5) disable system user