Hello,
We see the following error in the error log file
[09-Apr-2019 13:22:16 UTC] PHP Deprecated: Automatically populating $HTTP_RAW_POST_DATA is deprecated and will be removed in a future version. To avoid this warning set ‘always_populate_raw_post_data’ to ‘-1’ in php.ini and use the php://input stream instead. in Unknown on line 0
It could be related to your PHP version. We would recommend you to update PHP to 7.2 version. Let us know if you still have 404 error when editing the page after PHP version update.
Regards