I got the same message too.
1. Add Feed type
2. Select Processor as FILE
The message "Oops, something went wrong. Check your browser's developer console for more details." was displayed.
If continue to set up by "Save and add mappings", only "The website encountered an unexpected error. Please try again later." was displayed in a blank page.
In the Recent Log Messages, two entries was found.
Message 1
Warning: Undefined array key "class" in Drupal\feeds\Feeds\Processor\Form\DefaultEntityProcessorForm->getUpdateNonExistentActions() (line 299 of /var/www/html/osh/modules/contrib/feeds/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php)
Message 2
TypeError: class_implements(): Argument #1 ($object_or_class) must be of type object|string, null given in class_implements() (line 299 of /var/www/html/osh/modules/contrib/feeds/src/Feeds/Processor/Form/DefaultEntityProcessorForm.php).
Use F12 to check the browser console, other 2 error messages were found.
Error 1
Failed to load resource: the server responded with a status of 500 (500 Service unavailable (with message))
[{"command":"insert","method":"prepend","selector":null,"data":"\u003Cdiv data-drupal-messages\u003E\n \u003Cdiv role=\u0022contentinfo\u0022 aria-label=\u0022Error message\u0022 class=\u0022messages messages--error\u0022\u003E\n \u003Cdiv role=\u0022alert\u0022\u003E\n \u003Ch2 class=\u0022visually-hidden\u0022\u003EError message\u003C\/h2\u003E\n An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (1 GB) that this server supports.\n \u003C\/div\u003E\n \u003C\/div\u003E\n \u003C\/div\u003E\n","settings":null}]
Error 2
Uncaught
Drupal.AjaxError message : "\nAn AJAX HTTP error occurred.\nHTTP Result Code: 500\nDebugging information follows.\nPath: //admin/structure/feeds/add?ajax_form=1\nStatusText: 500 Service unavailable (with message)\nResponseText: The website encountered an unexpected error. Please try again later."
name : "AjaxError"
All are processors are fine except FILE. I am starting Drupal 10.0.7 and updated to 10.2.4. Feeds Version: 8.x-3.0-beta4. PHP 8.1 / 8.2
Feeds import usually used to data migration. Most of us installed it either the completion of site set up instead of with clean Drupal.
Any method could be trouble shoot easily?