I'm playing an Arche Seleukeia campaign too, and I just got the same problem (with Antiochos). I too suspect the Babylonian new year festival having something to do with it. The first new year I had it worked fine, had the -100% movement penalty for one turn, but the following new year I couldn't move him at all after the new year trait had (visually) gone.

If I have some time later, I'll take a look at what save games I still have and try to be a bit more specific about the circumstances.

BTW, is there any command for giving a character some movement points? Because poor Antiochos is sitting outside of town somewhere and can't even retire to a nice governor position without any movement points.