First off, plugins using PACE copy protection are evil. On top of PACE just being evil and user-hostile in general, it makes debugging the plugins impossible.

The plugin doesn't seem to accept any new notes when the kVstTransportCycleActive flag is set and no cycle start/end is specified, even if this is not technically required. OpenMPT does not set the kVstCyclePosValid, so those start/end times do not even need to be valid. The plugin seems to think otherwise, though. I have notified Sonivox but I somehow have doubts that they will fix their product if it works in all major DAWs - even if the fault is clearly on their side.

I don't have any problems regarding PACE copy-protection, or at least not with the only plugin that uses it (Xpand!2), which runs perfectly fine in OpenMPT, so this is probably more like either a vendor-specific or plugin-specific issue.

No, I don't mean that the bug is related to PACE - PACE is just an overly user-hostile copy protection (installs a driver into your system, may shut down the system if it detects "anomalies", and oh boy was that signup experience for an iLok account painful...)

Indeed. And this was actually the reason I reported the issue; before I weigh the option to leave this kind of software in my system, at least it should work properly. Well, if it is their fault, then...it is their fault.