This problem wasted a whole weekend for me. In case someone else experiences the same I post the solution here.
Symptoms: Cabbage plugins are found when scanning but when trying to include them on a track the FX window shows this error:
When a project using cabbage plugins is loaded the error looks like this:
Failed Attempts: The usual hint, delete plugin cache and rescan, does not help. Re-installing REAPER does not help. Re-compiling the plugins does not help.
C:\Users\...\AppData\Roaming\REAPER\REAPER.ini correct the line
lastcwd=C:\Program Files\Common Files\VST3\Cabbage to the directory where the Cabbage plugins are actually located and delete the line
vstpath64=C:\Program Files\Common Files\VST3.