This is our long but detailed reason for the app failure over the past 16hrs.
At 7.46 pm yesterday our server went offline for 5 mins, during this downtime our iOS user's apps attempted to gain access and failed, these requests continued to repeatedly fail, when the server came back online the queue of requests had stacked up and caused a memory full issue on our server and it again crashed.
This cycle of events kept happening until we blocked all-access, to all apps, at approx 11 pm last night, we attempted to reset our servers without the traffic and it came back online but each time we allowed app access it crashed again due to the overwhelming requests (1000's of requests per second)
This morning we found that during the server reset operation a critical file that allows external connections to our server had defaulted to OFF and we could not switch it on, our server hosts did this for us and we were then able to allow Android and iOS Testflight access again, during this time we were able to see in our logs 1000's of attempted connections from our iOS users and had to wait until these calmed down a little before attempting iOS full access again, had we done it any earlier there was a chance the server would have crashed again.
At 1.05 pm today we allowed access to iOS users and were able to keep the server stable enough that it never crashed under the load. Thankfully the server is behaving and the issues have been resolved (fingers crossed)