Unknown materials in the browser which I am not able to remove

[kaleen_pl]

Re: [Plugin][$] ThruPaint - v2.4a - 02 Apr 24

HI

We are using custom material folders (top folders) with ThruPaint, but have the issue that some unknown materials appear which we cannot get rid of. It seems they are some leftovers of material earlier renamed. When we delete the whole top folder with our custom material ThruPaint keeps showing them - as Unknown - see the pictures. Is there any Cache folder we should clear? Reinstalling / purging / Refreshing all material data did not help.