Thank erAck, I was unable to find a file of that name and the error went away by itself so I was able to add some test photos.
Then I installed the "user custom field" plugin and when I try to work out how to use it I get similar errors like these. Any idea if there's an issue with my setup, should it work on these?:
Environment
Piwigo 13.7.0 Check for upgrade
Installed on 25 June 2023, 5 days ago
Operating system: Linux
PHP: 8.1.2-1ubuntu2.11 (Show info) [2023-06-30 17:20:31]
MySQL: 8.0.33-0ubuntu0.22.04.2 [2023-06-30 17:20:31]
Graphics Library: GD 2.3.0
Cache size 5.59 Mo   calculated 4 days ago Refresh
Activated plugin list1
User Custom Fields

Create new custom fields

Edit custom fields
/var/www/photos/piwigo/_data/templates_c/p6jhns^db0e2c8d687818179cac362afeae772b0dcc7135_0.file.admin.tpl.php on line 253

Warning: Attempt to read property "value" on null in /var/www/photos/piwigo/_data/templates_c/p6jhns^db0e2c8d687818179cac362afeae772b0dcc7135_0.file.admin.tpl.php on line 253

Warning: Trying to access array offset on value of type null in /var/www/photos/piwigo/_data/templates_c/p6jhns^db0e2c8d687818179cac362afeae772b0dcc7135_0.file.admin.tpl.php on line 253