Click Appearance > Customize = 404 error, Click Appearance > Widgets = 404 error

I’m running WordPress 4.7.5 on a mature site that’s 3 years old. Since this latest update, I’m having 404 problems with these…

Click Appearance > Customize = 404 error
Click Appearance > Widgets = 404 error
Click Appearance > Menus = 404 error

When I Click Appearance > Themes or Editor = works normally

These were all working normally before updating to WP 4.7.5. Has anybody seen this or know of a solution?

I’ve been reading about this when I can find something. I’ve done these fixes with no results. The 404 error remains.

1.) Disable all plugins = 404 error remains.
2.) Change the theme to 2017 = 404 error remains.
3.) Re-save the permalinks = 404 error remains.
4.) Manually edit the .htaccess file = 404 error remains.
5.) Check the website links in wp-config.php which are OK = 404 error remains.
6.) Cleared the theme cache, cleared my browser cache and cookies = 404 error remains.

I even tried upgrading to WordPress 4.8 hoping that would fix it. Doing that expanded the 404 errors to the Admin area so I couldn’t access it to even log in. The log in page got stuck in a re-direction loop. I had to roll it back to 4.7.5 with a backup.

Normally I can figure these things out, but not this time. I have to ask for help.