AC800m high integrity (soft controller): forces on IO channels are immediately removed

I'm unable to force any signal (eg., RealIO/BoolIO) that is connected to an IO channel (AI880, DO880,..) via a faceplate. When I click "force" in the faceplate, I get the "Confirm Operation" dialog and click "Yes", then the channel very briefly becomes forced but the force is removed almost immidiately.

I don't have this problem if I disconnect the RealIO/BoolIO variable from the AI channel or if I change SIL level in the task and on the application to NON-SIL.

I have set the correct SIL Acess permissions. The "max number of forces" setting is also not the problem.

I run my application on a high integrity soft controller with simulated hardware. I've worked with AC800m HI controllers before and never had this problem. What could be wrong?