I just had this weird experience: on my Pantheon host, I installed a clean WordPress 4.7 and updated everything. Then I installed the WPMU Dev Dashboard and updated it too.
Since it is a clean install, I just reverted to the initial commit and retried. But to no avail.
To help you out, I will install the WPMU Dev dashboard WITHOUT the Appointments+ plugin. That way you will have access to the site.