I see this from time to time... verify you have a valid Operator (that could actually be called from that tenant) defined for all used tenants in Form 19, even if the tenant is not used for phones or anything else. It still pops up from time to time, easiest fix is a reboot but that isn't always the most convenient, it often fixes itself after a while or well playing with it using the Mitel PMS Emulator. Tech support was no help the first time I saw this, and I haven't bothered calling in on it again. Sorry, not much help, just seems to be a glitch in the system.