As per this bug report, I'm having an issue where I'm unable to save any changes to product attributes in the Magento 2 admin area.
When I try to change settings of any attribute and hit save, a POST request is made to (e.g.)
https://example.com/admin/catalog/product_attribute/validate/attribute_id/200/?isAjax=true but it receives no response.
The page then reloads to a more basic URL of
https://example.com/admin/catalog which results in a 404 error, and the changes are not saved.
There are no errors visible.
Please note I have tried clearing (and disabling) all caches, indexes, deployed static content, checked file/folder permissions, etc. and this is still happening.
Any ideas, please?