Suggestion to move the plugin folder outside of the protected
Program Files/Adobe/Adobe Lightroom Classic/ location worked. I compared the permissions on the plugin folder when under
Program Files and outside and the difference appears to be in the
Authenticated Users entry with
rwx permissions which is missing when the plugin is placed under
Program Files. Adding this permission to the plugin folder makes it work even when placed under
Program Files/Adobe/Adobe Lightroom Classic/.
Another approach that worked for me is placing the plugin under
C:\Users\<UserName>\AppData\Roaming\Adobe\Lightroom\Modules (as suggested also at Adobe Forums). Note that in my case the
Modules folder was missing and had to be created manually. After that though Lightroom has automatically found the plugin and it worked without errors.
There is still one other concern. Since my plugin was not registered yet on Windows, I tried to do that but the registration was not saved and the plugin was still in trial mode on the next Lightroom start. It only succeeded when starting Lightroom with Administrator priviledges and registering. It worked well with regular starts afterwards. This might be a part of the same permissions issue which this topic is about.