Backlight 6 Pangolin Album doesn't find download file

Hi all,
lately my download links under the image menu produce “The requested file was not found.” This text appears in a new tab named “Download Script”. The URL of the Tab points to https://kannznichkaufen.de/galleries/schafe-und-laemmchen-kurz-vor-ostern/download.php?file=photos/DSCF5693.jpg

In the Album template under “Advanced Settings” I selected “Use photos rendition”.
I tried switching this to “Disabled” and the download link disappeared as expected.
I switched it back, created a new album with the same template and uploaded a new photo, same problem.

In older albums created with older versions of Backlight the downloads links still work.
Have I broken something?

Thanks in advance and best wishes
Michael