Finally, I’ve isolated the issue and found a workaround.
I’ven’t had enough time to troubleshoot it all around, but this seems to be the cause. I use GIMP 2.8.22 + GMIC for GIMP 2.8.
When .xcf file is open with one or several layers in hidden mode and when a hidden layer is selected, GMIC does not launch, giving the above error gimp-item-is-group’: Procedure ‘gimp-item-is-group’ has been called with value ‘-1’ for argument ‘item’ (#1, type GimpItemID). This value is out of range.”
Every time I was opening the same file with hidden layer selected first.
Once a hidden layer is made visible - GMIC launches normally.
@David_Tschumperle I hope this can helpful as a bug report 
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Ah that is interesting indeed.
Anyway, if I create an image where I set a layer as ‘non-visible’, and run the plug-in on it, it is working for me (it just process the hidden layer as a normal layer).
So maybe I don’t understand what ‘hidden mode’ is ? Is it something different than ‘invisible’ ?
Could you provide eventually a .xcf file that causes the bug, and a quick description of the steps to reproduce the bug ? We are interested by fixing it, but right now, I’ve not been able to reproduce it !
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Will do
David, here’s the .xcf. file I was experiencing the issue with Dropbox - File Deleted
By hidden mode for a layer I mean a hidden layer, invisible.
I made a clean install of GIMP and GMIC from Kesselgulasch’s repository, launched the program, opened the file (layers were invisible), started GMICqt from Filters tab and the problem appeared.