Hello Paul.
Just tested here on my Ubuntu 20.04, with latest stable .deb package installed from the website (version 3.1.5), and I don’t get any error.
Could you tell me what G’MIC command the plug-in tries to run ? (using the “Paste Command to Clipboard” button located at the top-right of the filter parameter panel).
Also if you can provide an example of image that does not work.
If I can reproduce the bug here, I’ll probably be able to correct it (even though I’m not the author of this filter 
).
Thanks!