I am having the same problem on CUCM 11.5 with Genesys integration. When attempting to dial, the CUCM sends the above error to Genesys.
Not aware of a way to get a definitive list of situations that could cause it...my guess would be things like the device is off/disconnected/out-of-service or extension mobility logged out/in. Possibly trying to make a call when the max number of calls per line/device is reached..? If things like that seem unlikely, DevNet developer support should be able to assist in gathering/analyzing CTI/CUCM logs to determine exactly what happened - suggest opening a ticket: DevNet Support