You can work around this issue doing the following:.
1. Remove any vst3 plugins that get stuck from the vst3 folder. You can see which ones get stuck after reset and rescan all. You may need to repeat this step until all the offending VST3 plugins are removed.
2. Once you've removed the offending plugin(s), reset and rescan will allow you to get all the vst2 and AU plugins validated. VST3 format are the only ones that cause problems.
3. Once this is done, add back one of the offending plugins and then click scan for changes. Repeat this step until you have all of your plugins added.
step 3 will sometimes correctly scan the offending plugins. Sometimes it will still fail. If it gets stuck in a loop again you'll need to start over from the beginning, and just leave out the ones that don't work.
I was able to get Kontakt and Guitar rig to scan correctly using this approach. Those are the only two NI plugins I have.
I also needed to remove Melodyne, TXLtimecode, and some of the spitfire plugins.
If you end up needing to leave out the VST3 version, you can use the others in the mean time until this issue gets fixed.