The IDoc handling between MII 12.1 and MII 12.0 is completely different at the technical level. So not sure if the fix for MII 12.0 would affect/fix the problem in MII 12.1. However MII IDoc Processing in 12.1 is heavily dependent on JMS. I had seen some instances where the JMS infrastructure of NW 711 on Windows Server. It may cause processing to get stuck. I think this was solved with NW 730. You may try it if this is a feasible solution.
Regards
Partha