Just for testing purposes: does it make any difference when launching Reaper as admin (via Right-Click > Run as administrator)?
If this doesn't make any difference, I'd suggest to try the following next:
1a) In Reaper go to Options > Show REAPER resource path ...
1b) Or open the resource path (C:\USERS\<username>\AppData\Roaming\Reaper) directly in the Windows File Explorer
2) Delete the 'reaper-vstplugins64.ini' file in this folder
3) Restart Reaper
4) Maybe also do another [Clear cache/rescan], just in case
Otherwise try this:
1) Uninstall Kontakt (only the Kontakt Player - not the Factory Selection):
https://support.native-instruments.c...ndows-Computer
2) Then use this tool to remove any 'Kontakt' entries from the registry:
https://support.native-instruments.c...cts-on-Windows
3) Restart your computer afterwards and reinstall the Kontakt Player again via Native Access