ios CoreBluetooth[WARNING] Unknown error: 1309

That's a known issue, It's caused due to deadlock in CoreBluetooth (Apple's bug),1309 error is mostly appears when your app operates as a Central and Peripheral, and when the operations are overlapping each other, in that case deadlock will be produced (which can be resolved by rebooting device).

Seems BLE stack gets corrupted in some other cases too (iOS 7 and lower), on iOS 7.1 stack is much more stable, and doesn't have issues like this.
How we resolve issues like this?
Showing troubleshoot screen where user can fix problem himself/herself.

You can find known iOS issues here http://help.getpebble.com/customer/portal/articles/957568-troubleshooting#Pair

Anyway I think you can start using https://github.com/l0gg3r/LGBluetooth which will reduce chance having bugs on your side, and make your job much more effective.
Here are read/write examples

Read

[LGUtils readDataFromCharactUUID:@"f045"                     serviceUUID:@"5ec0"                      peripheral:peripheral                      completion:^(NSData *data, NSError *error) {                          NSLog(@"Data : %s Error : %@", (char *)[data bytes], error);                      }];

Write

int8_t dataToWrite = 0xFF;[LGUtils writeData:[NSData dataWithBytes:&dataToWrite length:sizeof(dataToWrite)]       charactUUID:@"cef9"       serviceUUID:@"5ec0"        peripheral:peripheral         completion:^(NSError *error) {            NSLog(@"Error : %@", error);        }];

Since the CoreBluetooth is issuing this “warning”, there has to be someone at Apple or in the Bluetooth community who wrote the code or at least knows what situation is triggering the message. Can anyone tell me what is causing this and what I should do to avoid it, or point me to some material that will help me in this endeavor?

If I had to hazard a guess, I’d say that the primary reason you haven’t received much of a response is because you’ve given no description of your issue beyond “sometimes I see error 1309 and things stop working”.

If you haven’t already, please file a bug on bugreporter.apple.com and include as much detail as possible - a description of what you’re trying to do, repro steps that lead to the issue, even a test app. Respond to me directly with the bug number, and I will follow-up