Writing a 3rd party plugin: Custom policy file sanity checks failed

I am developing a 3rd party plugin. Since I need write access to the file system and I do not want to switch off the protected mode I added a policy for my plugin.

When starting Acrobat, the log files of Adobe Acrobat Protected Mode Logging says:

Custom policy file sanity checks failed: C:\Program Files\Adobe\Acrobat DC\Acrobat\MyPlugIn_policyconfigfile.txt

When I add the "standard" policy file ProtectedModeWhitelistConfig.txt everything is fine. The logging says:

Found custom policy file: C:\Program Files\Adobe\Acrobat DC\Acrobat\ProtectedModeWhitelistConfig.txt

Adding custom policy: FILES_ALLOW_ANY = d:\MyPlugInFiles\*

It makes no difference if MyPlugIn_policyconfigfile.txt even exists or is empty. The error message remains the same. It can't be a syntax issue either, since the same policy works fine in ProtectedModeWhitelistConfig.txt.

I also checked the read access to MyPlugIn_policyconfigfile.txt which is ok.

Did I overlook an additional switch to enable 3rd party plugins?