We moved the private_files folder to another location (including the mailsystem subfolder). This results in a blank website. As soon as the mailsystem (including *inc files) are put back in original location the website is working again.

The new location of the private_files folder is changed at http://{mydomain}/admin/config/media/file-system

Is the path to the mailsystem subfolder containing the *inc files hardcoded in the mailsystem module?
Other modules seems to work fine with a other location for the private_files folder.