iOS9 'modifying the autolayout engine from a background thread' without using autolayout - ios

Question

I started getting this warning :
This application is modifying the autolayout engine from a background
thread, which can lead to engine corruption and weird crashes. This
will cause an exception in a future release.
Here is the backtrace with the warning message :
Stack:(
0 CoreFoundation 0x25542303 <redacted> + 150
1 libobjc.A.dylib 0x24d0edff objc_exception_throw + 38
2 CoreFoundation 0x25542231 <redacted> + 0
3 Foundation 0x25e25bbb <redacted> + 170
4 Foundation 0x25ccb637 <redacted> + 38
5 UIKit 0x297e0431 <redacted> + 52
6 UIKit 0x297e0e1f <redacted> + 222
7 UIKit 0x297fd52d <redacted> + 96
8 UIKit 0x29efe579 <redacted> + 320
9 UIKit 0x299dc8e9 <redacted> + 148
10 UIKit 0x299cb44f <redacted> + 42
11 UIKit 0x296d5a83 <redacted> + 714
12 QuartzCore 0x277b1ad5 <redacted> + 128
13 QuartzCore 0x277ad1d1 <redacted> + 352
14 QuartzCore 0x277ad061 <redacted> + 16
15 QuartzCore 0x277ac581 <redacted> + 368
16 QuartzCore 0x277ac233 <redacted> + 614
17 QuartzCore 0x277d9b63 <redacted> + 310
18 libsystem_pthread.dylib 0x25279905 <redacted> + 508
19 libsystem_pthread.dylib 0x25279507 <redacted> + 86
20 libsystem_pthread.dylib 0x2527a485 pthread_exit + 28
21 Foundation 0x25ca31d7 <redacted> + 10
22 Foundation 0x25d5e34f <redacted> + 1178
23 libsystem_pthread.dylib 0x2527ac7f <redacted> + 138
24 libsystem_pthread.dylib 0x2527abf3 _pthread_start + 110
25 libsystem_pthread.dylib 0x25278a08 thread_start + 8
As far as i know i don't use Autolayout (as i want the app to be iOS 5.1.1 compatible). and also i don't seem to be within the reported backtrace.
I also have the PSPDFUIKitMainThreadGuard class enabled, which should be checking the main thread accesses but nothing trigered out of this thing.
Is there any way how to find out what is doing such a thing?
Question This application is modifying the autolayout engine from a background thread, which can lead to engine corruption and weird crashes doesn't explain how to debug such thing.

Answer 1

Whether or not you are using Autolayout directly is not the issue at hand. The OS is converting legacy autoresizingMask to modern NSLayoutConstraint under the hood.
The issue has to do with modifying the UI from anywhere but the main tread.
This typically happens when:
responding to a NSNotification
returning from an async method
executing a completion block on a thread you do not control
explicitly scheduling a task in the background
Note about The app doesn't crash...
The app will crash. All you need is time and a few thousand users...