Is it possible your hub's IP address changed and you're trying to use the old one? Go to http://findmyhub.hubitat.com to see what you find there. Of course, if you got to the Diagnostic Tool or the local rather than cloud Dashboard, that would suggest you're using the correct IP, but perhaps you're using a different method of access for those...
As stated above, its a known issue with iOS beta 16.4
Not sure if its an apple bug or some change in the HTML spec Hubitat needs to comply with yet. I would guess it is a bug in the Apple iOS, usually html changes do not break entire pages.