How can error pages be customized

I believe your problem stems from the upload process itself. If so, you do not need to customize an error message. Instead, you need to update your upload settings, and/or publish your file size limits to your community. This article describes what causes that error, and how to address it. Good luck!

502 Error when edit some users

It is most likely that your server is running out of resources. It happens randomly. I would recommend you contact the hosting provider and show them that error. Also ask them to increase the PHP memory limit on your server and look into the server error log files. The 502 error is too generic, it … Read more

mixed contents admin panel is Unsecured

That should not effect wp-admin, I have never seen that happen before. Are you sure both your site url and homepage url’s are set to https:// If you really want to change them to see if that is the issue, you could edit them in the following files: (there are multiple mentions of that url … Read more