"Decoding failed with error code -1" from Objective-C Program running under Xcode

I occasionally am getting these kinds of error messages from my Objective-C program on a Mac:

2018-05-07 10:35:37.127742+0200 ScrumPLoPBookBinder[7679:2276295] [0x102885600] Decoding failed with error code -1
2018-05-07 10:35:37.127809+0200 ScrumPLoPBookBinder[7679:2276295] [0x102885600] Decoding: C0 0x013100E5 0x0000304A 0x22111100 0x00000000 2778
2018-05-07 10:35:37.127834+0200 ScrumPLoPBookBinder[7679:2276295] [0x102885600] Options: 305x229 [FFFFFFFF,FFFFFFFF] 0001C060

Sadly, there is no apparent ill effect on the program output from which I can track the way back to this error. It happens during some rather complex computation so it is difficult for me to even start to localize it to any code, compounded by the fact that it is sporadic. I have not been able to create a test that repeatedly reproduces it.

I vaguely remember being able to breakpoint for all internal error messages, but can find instructions only for how to track down exceptions. Can someone help me find what is causing this problem?

XCode Version 9.3 (9E145), MacOS 10.13.4, MacBook Pro.

I'm seeing it occasionally (but on iOS) when AdMob loads a new banner.