Change in the area of coroutine handling in since b14?

Our main project was working fine in b13 and then with no changes started crashing in b14. After many hours working through it, it seems to be when a coroutine yields during startup, a menu then starts to load and then you get a crash. If you stop the menu loading, all is fine. The project is huge so have not submitted a bug report yet, but it’s still happening in b16 :o(

Any functional changes that might cause this? Sound familiar to anyone?

After b13 everything went to crap. Crashes constantly, definitely seems to be around corountines, though is not limited to start-up.

No bug report with small repro to track?