Hello Matthias,
this ticket is not about the cause of the error but more about the quality of
the error message. I am not sure if Debian or upstream is responsible.

Here are two errors (from
https://appstream.debian.org/sid/main/issues/backintime-qt.html).

screenshot-save-error: Unable to store screenshot for
'https://translate.codeberg.org/media/screenshots/bit_16to9_960x540_mainwindow.png':
Could not load source screenshot for storing: Unrecognized image file format
screenshot-save-error: Unable to store screenshot for
'https://translate.codeberg.org/media/screenshots/bit_16to9_1280x720_general_ssh.png':
Could not load source screenshot for storing: Unrecognized image file format

I am not 100% sure but the technical reason might be that the source hoster was
down at this time or the hoster blocked the request assuming Debian was an AI
crawler.

But error message states something different. Users might think the file is
invalid, wrong format or PNG is not allowed in AppStream meta data.

I am just the upstream maintainer of "backintime". I am sorry to bother you
with this. Usaly the DM should by first contact. But they are not responsive
and/or often state not having resources diving into such details.

Regards,
Christian Buhtz