This is really annonying. And I don’t have any explanations right now.
I’d be really interested to have more information. Reproducing the bug on my machine is probably the only way I can fix it (particularly right now, where I’m confined at home).
Would it be possible to open the plug-in, go to ‘Settings → Other → Output messages’ and set it to ‘Debug (log file)’ ? Then try to run your filter (so get the error), and send me the log file produced by the plug-in, that should be available in your folder %APPDATA%/gmic/ (if I remember well 
).
This could help understanding the problem! Thanks.