Salut David,
Problem solved - via a workaround.
I installed gmic_gimp_qt 3.1.5 and descreen works fine in the “official” Gimp that comes with Xubuntu 20.04 (v2.10.18), as it did with the earlier version gmic-gimp 2.4.5 (the one from the repos).
Only gmic 3.1.5 shows the button to copy the command to the clipboard, v2.4.5 and the gimp appimage with filters (gmic 2.9.2_pre#…) do not show that button.
The command in question is fx_pahlsson_descreen 0.
In the appimage (gimp 2.10.25) descreen produces another error msg:
I solved the whole problem by using gmic descreen from the commandline, I made a script to batch process several files, as this filter is a bit time-consuming.
Thanks for your time anyway.
Regards, Paul.