Certain VSTi's seem to have disappeared from the FX Browser in 3.76 in an unexpectedly impromptu manner. Specifically, Wusik EVE (I'm told that MOTU Mach Five 2 has similarly disappeared - though its DXi install remains visible but not as useful as the DXi version is not as stable). This is (as far as I am aware) with no change to Reaper's preferences.
It could also be that this problem initially appeared in a previous 3.xx release and I just didn't notice until now.
Strangely, Wusik EVE DOES appear as normal in Reaper 4 beta 7 (though Mach Five II does not).
EDIT: Sorry, scrap the thing about Mach five II - apparently this was user error.
The problem with Wusik EVE remains though.