Greasy Fork is available in English.
Downloading is possible for both photos and videos from posts, as well as for stories, reels or profile picture.
This is wrong photos position cause it.
And your problem just a coincidence.
So... That error has been fixed in version 2.0.18.
Please try again at latest version!
on Firefox direct download is not working it just opens an about:blank tab when clicking download arrow.
anyway thanks.