I have just determined with 100% certainty that your domain mapping plugin has an issue with the latest version of wordpress. I couldn’t get pages to load, so I deactivated all network plugins. I had a hunch, mapping was the plugin. When it was activated the issue reoccurred. I have granted support access so you can see the issue 1st hand.
My issue is that I have clients who need their domains working properly. Right now they are all resolving on subdomains.