Assuming this happens only on a Towabarless tug that raises the airplane, it might be caused by the airplane not allowing to turn the engines on, because GSX has frozen it and put it in a slightly pitch-up attitude, which might be interpreted by the airplane code as "flying", so there might be some custom code preventing to start an engine in that situation.
If this is the case (I can only guess), only the developer can fix it, by recognizing GSX is still pushing, which is clearly documented in the developers sections of the manual.
