For anyone else reading this, You most likely want to choose (very verbose log file) to get useful results for troubleshooting. Plain (verbose log file) wasn’t very verbose with only 2 lines, and debug created a file almost 4mb in size.
Here are the results:
[gmic]-0./ Start G'MIC interpreter.
[gmic]-0./gui_filter_sources/ Skip argument '1,1'.
[gmic]-0./gui_filter_sources/*if/ Start 'if...endif' block -> condition '1' holds.
[gmic]-0./gui_filter_sources/*if/ Set local variable 'local=/home/akovia/.config/gmic/update291.gmic'.
[gmic]-0./gui_filter_sources/*if/*if/ Start 'if...endif' block -> condition 'isfile(['/home/akovia/.config/gmic/update291.gmic'])' holds.
[gmic]-0./gui_filter_sources/*if/*if/ Import commands from file '/home/akovia/.config/gmic/update291.gmic' (2013 new, 2185 replaced, total: 4171).
[gmic]-0./gui_filter_sources/*if/*if/gui_filter_sources/ Input image at position 0, with values (104,116,116,112,115,58,47,47,103,109,(...)116,101,50,57,49,46,103,109,105,99,1) (1 image 31x1x1x1).
[gmic]-0./gui_filter_sources/*if/*if/gui_filter_sources/*local#42849/ Input file '_gui_filter_sources' at position 0
[gmic]-0./gui_filter_sources/*if/*if/gui_filter_sources/*local#42849/ e[0;31;59me[1m*** Error (file '/home/akovia/.config/gmic/update291.gmic', line #42849) *** Unknown command or filename '_gui_filter_sources'; did you mean 'gui_filter_sources'?e[0;0;0m
[gmic]-0./gui_filter_sources/*if/*if/gui_filter_sources/*local#42849/ Remove image [] (0 images left).
[gmic]-0./gui_filter_sources/*if/*if/gui_filter_sources/*local#42850/ e[0;31;59me[1m*** Error (file '/home/akovia/.config/gmic/update291.gmic', line #42850) *** Unknown filename '/home/akovia/.config/gmic/gui_filter_sources'.e[0;0;0m (1 image 18x1x1x1).
[gmic]-2./gui_filter_sources/*if/*if/ Return.
[gmic]-2./ End G'MIC interpreter.
[gmic_gimp_qt]./preview/ v 3 fx_pahlsson_descreen_preview 0
[gmic_gimp_qt]./preview/ v 3 fx_pahlsson_descreen_preview 0
[gmic]-1./ Start G'MIC interpreter.
[gmic]-1./ Abort G'MIC interpreter (caught abort signal).[gmic]-1./ Start G'MIC interpreter.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Import custom commands from expression '_split_preview : -fx_pahlsson_descreen 0 k[0]' (1 new, total: 4172).
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Set local variable 'is_movable=0'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Set multiple variables 'posx=50', 'posy=50'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Set multiple variables 'pw=376', 'ph=834'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/*repeat/*local/ Set local variable 'is_failed=0'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/*repeat/*local/*local/ Apply command '_split_preview' on image [0], with a timeout of 015 seconds.
[gmic]-2./fx_pahlsson_descreen_preview/gui_split_preview/*repeat/*local/*if/ Keep image [1] (1 image left).
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Discard definition of custom command '_split_preview' (1 found, 4171 commands left).
[gmic]-1./ End G'MIC interpreter.
[gmic_gimp_qt]./apply/ v 3 fx_pahlsson_descreen 0
[gmic]-1./ Start G'MIC interpreter.
[gmic]-1./fx_pahlsson_descreen/ Skip argument '0'.
[gmic]-1./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/ Split color and opacity parts of image [0].
[gmic]-1./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Force image [0] to be in RGB mode.
[gmic]-1./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Convert color representation of image [0] from RGB to CMYK.
[gmic]-1./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Split image [0] along the 'c'-axis.
[gmic]-4./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Set name of image [0] to 'C'.
[gmic]-4./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Set name of image [1] to 'M'.
[gmic]-4./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Set name of image [2] to 'Y'.
[gmic]-4./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Set name of image [3] to 'K'.
[gmic]-4./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Append images [0,1,2,3] along the 'c'-axis, with alignment 0.
[gmic]-1./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/*local/ Convert color representation of image [0] from CMYK to RGB.
[gmic]-1./fx_pahlsson_descreen/pahlsson_descreen/*repeat/*local/ Append image [0] along the 'c'-axis, with alignment 0.
[gmic]-1./ End G'MIC interpreter.
[gmic_gimp_qt]./preview/ v 3 fx_pahlsson_descreen_preview 0
[gmic]-1./ Start G'MIC interpreter.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Import custom commands from expression '_split_preview : -fx_pahlsson_descreen 0 k[0]' (1 new, total: 4172).
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Set local variable 'is_movable=0'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Set multiple variables 'posx=50', 'posy=50'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Set multiple variables 'pw=376', 'ph=834'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/*repeat/*local/ Set local variable 'is_failed=0'.
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/*repeat/*local/*local/ Apply command '_split_preview' on image [0], with a timeout of 015 seconds.
[gmic]-2./fx_pahlsson_descreen_preview/gui_split_preview/*repeat/*local/*if/ Keep image [1] (1 image left).
[gmic]-1./fx_pahlsson_descreen_preview/gui_split_preview/ Discard definition of custom command '_split_preview' (1 found, 4171 commands left).
[gmic]-1./ End G'MIC interpreter.
So this was a good way to see all the old or corrupt faves, so I was able to remove them all. Unfortunately, it made no difference to this descreen problem. I see the errors, but not sure what to make of them.
I did notice quite a few empty files in my .config folder. Here is a directory listing with file sizes.
$ tree -s
.
├── [ 5641] gimp_faves
├── [ 1997151] gimp_update1652.gmic
├── [ 1977113] gimp_update1661.gmic
├── [ 1996189] gimp_update167.gmic
├── [ 2123146] gimp_update168.gmic
├── [ 2137458] gimp_update169.gmic
├── [ 2149997] gimp_update170.gmic
├── [ 2193537] gimp_update172.gmic
├── [ 0] gimp_update.lock
├── [ 16732] gmic_qt_faves.json
├── [ 5358] gmic_qt_log
├── [ 7390] gmic_qt_params.dat
├── [ 7415] gmic_qt_params.dat.bak
├── [ 63] gmic_qt_visibility.dat
├── [ 72037] grain_kodak_tmax3200.cimgz
├── [ 81739] grain_kodak_tmax400.cimgz
├── [ 71813] grain_orwo_np20.cimgz
├── [ 1316] hotchocolate.gmic
├── [ 177832] sample_barbara.png
├── [ 371659] sample_landscape.png
├── [ 1384219] style_greatwave.png
├── [ 1428730] style_landscapenearantwerp.png
├── [ 1340166] style_leviaducalestaque.png
├── [ 1140474] style_littlebayatlaciotat.png
├── [ 1424579] style_portraitdemetzinger.png
├── [ 1623330] style_themandola.png
├── [ 1631883] style_windowsopensimultaneously.png
├── [ 0] update222.gmic
├── [ 0] update223.gmic
├── [ 0] update230.gmic
├── [ 0] update231.gmic
├── [ 0] update232.gmic
├── [ 0] update233.gmic
├── [ 0] update235.gmic
├── [ 0] update236.gmic
├── [ 0] update240.gmic
├── [ 0] update241.gmic
├── [ 0] update242.gmic
├── [ 0] update243.gmic
├── [ 0] update244.gmic
├── [ 0] update245.gmic
├── [ 0] update253.gmic
├── [ 0] update261.gmic
├── [ 0] update263.gmic
├── [ 0] update267.gmic
├── [ 0] update270.gmic
├── [ 0] update275.gmic
├── [ 0] update280.gmic
├── [ 0] update281.gmic
├── [ 0] update282.gmic
├── [ 0] update283.gmic
├── [ 0] update284.gmic
├── [ 0] update290.gmic
└── [ 4370036] update291.gmic
Thanks