Can you tell me anything more about the state of the unit?
I've seen RTE 5 on changing unit mission occasionally --
* When changing from Withdrawal to Hasty Defence, when the unit was in Retreat Mode and combat ineffective.
* When changing a unit's HQ.
In the former case, restarting from a savefile (autosave.btl) enabled me to get around the problem. [Autosave.btl is saved just before executing the turn; autosave2.btl is saved just after executing a turn.]