I've had this problem only since upgrading to AutoIt 3.1.1. I had no problems with other 3.x versions. I had a short script compiled in an older version and using the runasset command worked fine; with no other changes, I recompiled it today with v3.1.1 and then got the same error as mentioned above. Adding the @systemdir as the working directory worked for me.
I assume that works because @systemdir is the location of the windows runas.exe command??? Anyone know if that is correct?