Filebrowser thumbnails "can not create img for private://"
This is a workaround I used to solve the issue:

[https://www.drupal.org/project/private_files_download_permission]

I don't consider this 'magic' to be a fix. There is a lack of documentation and/or a bug in the filebrowser module.

How can one raise an issue with the developers?

PS: Also, now have another issue with the filebrowser module: sort order is not being honoured in Grid view!