I had the same problem despite the fact that the plugins existed in the ‘plugins’ directory (C:\wm65\Developer\plugins).
What had happened was that I renamed the installation directory, changed the name in the ‘developer.bat’ file but forgot to change it in the plugins ‘ini.cnf’ file.
Then when I started Developer, it couldn’t find the plugins that were specified in the ‘ini.cnf’ file and so removed them from the ‘ini.cnf’ file.
The contents of the ‘ini.cnf’ file should look something like this (all on the one line):