I am having this exact problem when trying to move creeps south from E18N11 to E18N10 or from E19N11 to E19N10. I can successfully move creeps between my rooms E18N11 and E19N10 but cannot go south for some reason. The creeps seem to blink between the rooms along the exit as described by the OP. Today the novice area walls were removed from the middle of E18N10/E19N10, not sure if that is relevant. I've tried both moving a creep to a flag in the target room as well as moving them to the exit specifically using the same code the OP posted. Neither worked. 

I have just tried to move laterally out of my room and have encountered the same problem. I am now stuck in my 2 rooms...