In order to experiment with LUTs I changed darktable’s preferences to use a specific LUT folder (options, processing, LUT 3D root folder).
Darktable appears to regularly page that folder (and in my case it sometimes complains about invalid LUTs).
I am unlikely to use LUTs on a regular basis and would like to reset that preference to the default, but I cannot work out how to do this.
As a workaround I have changed darktable’s preferences to use an empty folder.
That is the “correct” workaround.
I had the same issue in the past after I tried to work with LUTs.
The default used in a fresh install is an empty entry. To use the LUT3D module you have to set the preference to the folders location. After you have set your LUTs location folder entry, you can’t set the entry to empty again. There is no way to do this from the folder selection widget.
The selected folder entry is used to build a list of all files with known LUT file extensions in the selected folder and its sub folders. These files are then analysed and if there are errors in a file the “invalid LUT” message is shown. If either the entry is empty or points to an empty folder, an empty file list is returned and no file check happens, so no “invalid LUTs” message will be shown.
Many thanks for your prompt and helpful response!