XMLHttpRequest Error When Accessing phpMyAdmin from WordPress Site URL

I’m currently hosting a WordPress site. When I attempt to log in to the phpMyAdmin using the site URL (i.e., https://www.MyWordPressSite.com/phpMyAdmin) I get the following error message:

Error in processing request
Error text: TypeError: Object doesn’t support property or method ‘setRequestHEADer’ (rejected)
It seems that the connection to server has been lost. Please check your network connectivity and server status.

I also have a Concrete5 site running on the same server. When I access phpMyAdmin using that site’s URL (i.e., https://www.MyConcrete5Site.com), I have no issue, despite the fact that they point to the same code. I’ve already attempted to disable all plugins and revert to a released WordPress theme, and didn’t have much luck.

Any ideas? Is there a reason phpMyAdmin would behave fine from the C5 URL, but throw an XMLHttpRequest error when accessed from the URL hosting a WordPress site?

The page I need help with: [log in to see the link]