With Reaper closed
, try looking for the file reaper.ini (it may be in a hidden folder).
Open it in a text editor and find the line that starts "vstpath=". Edit that to include only a path to (some of ) your VST plug-ins. Save it and start Reaper.
You should now see (some of) your plug-ins in the FX Browser. If not all of them are listed, you'll need to add / edit the VST paths. Click [Options]. select "Preferences" >> Plug-ins >> VST and Edit them. Be as precise as you can, so you do not include unnecessary paths.
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You might be able to just delete that line in reaper.ini and start Reaper. I do not know, but there's no harm trying.