Just gonna chime in here and say of all the different "fixes" that have been recommended (which none seemed to work for me no matter what I tried), I accidentally resolved the scanning issue by simply turning off my wifi. All plugins scanned perfectly, after initial scan, turning back on it will work as normal (although if you add a new vst it hasn't scanned yet, it might fail again in which case try turning off the wifi again). Not sure why this worked but that's definitely what did it for me. Running on OSX Mojave.
