I have spent the entire day trying to fix what I do believe by now it is a bug from Apple'e end.
I have successfully uploaded an iOS build to iTC which has been properly processed and states 'testing' under the internal testing tab of the TestFlight section.
I tried switching Internal Testing on/off in vain (always gives 'operation_failed'). I also tried to remove and re-add internal testers.
No invites sent. Nothing on TestFlight. What the hell is going on?
Note that this isn't the first build I submit for internal testing. Others used to work normally (Back in XCode 7.0 and the older version of iTC)