Your log shows you don't have the latest update installed, because it refers to a line that has been moved so, first thing to do would be updating.
However, the error seems to indicate something very strange happened, because it suggest the whole pushback truck already disappeared when it was time to confirm a good engine start.
However, since in the latest update we changed exactly that ( vehicles won't be removed until the marshaller walked back to the gate ), it's likely this error shouldn't happen anymore.