Just installed 12.3.0 and uploaded my first image. I thought all was well until I went to view the image and received an error similar to:

Undefined array key "path_ext" in the picture_content_asize.tpl.php

After a bit of digging, I found that the "modus" plugin was in need of an update. The fix was to go to the Admin page> Tools> Updates> Extensions Update and update the modus plugin.

This one was definitely the source of the issue since it was the only plugin installed.

Piwigo 12.3.0
Operating system: WINNT
PHP: 8.0.0
MySQL: 5.5.5-10.6.5-MariaDB
Graphics Library: ImageMagick 7.1.0-41