I've made a song which is using two mda Degrade VST effects.
(the first was set to Channels 1 and 2, and the second to Channel 3)
Adding one seemed to work fine, but when I added the other OpenMPT flooded me with error messages like:
"The plugin mda Degrade threw an exception in processReplacing. It has automatically been set to "Bypass"."
and then lots of:
"Exception in ProcessVSTEvents() (Plugin=mda Degrade, pEventList:00C3B818, numEvents:64, MidicodeCh:0, MidicodeEv:224, MidicodeNote:0, MidicodeVel:64)"
"Exception in Dispatch(72) (Plugin="mda Degrade")!"
"Exception in Dispatch(12) (Plugin="mda Degrade")!"
"Exception in Dispatch(

(Plugin="mda Degrade")!"
(the last one at least 3 times)
And when switching between "Patterns" and "General" view I get more of them, this time with "Dispatch(7)" and "(6)".
And when shutting down the program I get an "Dispatch(1)" error.
This also happened in OpenMpt .39, and the old closed source MPT crashes without errors (though I know I've used multiple "mda Degrade" effects in songs in some older versions of closed MPT.)