When you are in the status report of the admin area are you finding the following issue:
ENTITY/FIELD DEFINITIONS Mismatched entity and/or field definitions The following changes were detected in the entity type and field definitions. File The File type field needs to be installed. Paragraph The Published field needs to be updated. The Behavior settings field needs to be updated.
Check whether you have the module file_entity. As File Entity has mostly been installed in to the core, this could be the fall out of different versions. On the module project page there is a note
This module has been renamed or deprecated by another module in Drupal 8
If you uninstall the module, then the error will vanish.
For some, resolving the error will be uninstalling the module. However, if you were like us, the uninstall option failed to work. So to test that this is the error (for us), we rebuilt a relative new site without the module. And presto, the error was gone.
More on the error... when reviewing the server logs the error was:
Uncaught PHP Exception Drupal\\Core\\Field\\FieldException: "Attempt to create a field field_image_alt_text that does not exist on entity type file." at /var/www/vhosts/example.com/dir/core/modules/field/src/Entity/FieldConfig.php line 293, referer: https://example.com/admin/modules/uninstall/confirm