For admins I would assume using /op again would remove their admin role or even /deop, if not /role [rolename] privilege [grant|revoke] [privelegename] should work as well. (I am not well versed in server commands take this with a grain of salt.) As for the error it's a null reference exception meaning you used that command that then pointed to nothing and it backfired the server. If you aren't updated to the latest version of the game please do so. If you already are and still have this then scroll up, click need help? and make a ticket under vintagehosting or wait until someone that is more technical with code than I am to come by.