Now it hit me, too
On one machine, I needed to reinstall everything - including Windows 10.
Of course I copied the "normal" VST plugin Dlls to the same directories as before, and (seemingly successfully) installed the Native Instrument stuff using "Native Access".
I added several locations to the VST search path.
"Clear and Re-Scan" resulted in Reaper just closing during the scan and no change otherwise.
After some experimenting I found that the closing happens when a dell with the same name is found in multiple folders in the search path.
-Michael