Seen that error today, it happened after I upgraded XCode to 7.0 – in lower version everything was ok (well, it was not but just I did not saw it). So what does it means? Somewhere in your code you are changing something that affects UI – this can be button or label text or just table data, doesn’t matters. What is important – you should not do it in background. Solution is simple just use:
// put your code here
Changing any UI stuff in background can work or cannot. You never know, it is always a good practice to put this on main thread.