I am getting this same error, but only when posting to a 10.2 Print Service (Execute Task (Export Web Map Task)). The same data posted to 10.1 works. Also, if I do a get with the same data on 10.1 or 10.2 it also succeeds, so it seems that there is some issue with a post against 10.2.
Anyone find any fix, workaround or any suggestions?
Thanks
Simon