A solution was found, see bottom of post
To update my mods to KSP 1.8.0 i need to recompile all my KSPedia entries with the latest PartTools and Unity 2019.2.2f.
For this purpose i made a fresh install of Unity 2019.2.2f, uninstalled preloaded TextMesh Pro, installed the TextMesh Pro specified and afterwards installed the latest Parttools. Exactly as explained here:
It seems that compiling the KSPedia slides is impossible with the PartTools. After installing them i get