Hi purple,
the message is from your Browser. Its about TLS and the Certificate… yes it can be a Men in The Middle issue solved by certificate pinning.
Or if you know the last and the new certificate and trust it, when it got a renew or you had just old Information in your cache.
On my Computers this was just a one time issue solved by reloading the Webpage. And no i am not use ATT directly. Had that error Message too with the forum or some other like popular News pages.
I thought it was triggered by Content from a third IP, on some Webpage and cause i use Adblockers and Script-Blockers which block some Domains.
Could also be some kind of update issue, if some Files or Databases are not ready to deliver Information on that server cause the Database restart or a network files system was not ready and triggered the End of File Error.
Its just a Guess. Try to reproduce and you will see that the Error is kind of volatile.
However there is a small to medium chance that some-one/thing try to force you on an Men in the Middle Attack.
Just check and note the IP (which can change quite often) and or the Certificate and when it runs out to be renewed automatically…
Good Luck!
Cristal