i found a work around. all you need to do is leave one unit at Tripoli. the convoy will say it suppling only one unit, but is seem to spead out to any others.
Good workaround, however, at the start of the Barbarossa scenario, and probably others, the German N. Africa units are close to Tobruk, far away from Tripoli. It will take four turns for me to get a German unit into Tobruk, by sea transport.
However, your workaround should certainly give the programmers a clue as to how to fix this, or what the exact problem is.