This might have some useful info: Renoise 3.4 does not load a VST that worked in 3.2, 3.3 - #12 by James_Britt
Basically, if I understand it right, Renoise will track plugins that failed to load and stop showing error messages about them. When I deleted CachedFailedVSTs_x64.db (and the other related db files with similar names) and then re-opened Renoise I again saw messages about what was failing.
If you grab a tool for browsing SQLite databases you can open the files and see the list of VSTs and their paths