New crash on Forerunner 735xt

My app has been running on the 735xt, and it has always run on the simulator with the exception of the recent problem with the simulator displaying Menu items incorrectly.

My most recent build crashes on a real 735xt before displaying the first screen.

The customer provided the crash log.

ERROR: Too Many Arguments Error
DETAILS: Failed invoking <symbol>
STORE_ID: 791191d40593426c838dee71558a967d
CALLSTACK:
@PC = 0x30001e91
@PC = 0x1000412d
@PC = 0x10000988

I've converted the hex addresses to decimal and gone into the debug xml file.

The first two entries on the call stack decode to logical locations in my code.

The last entry, 0x30001e91 does not show up in the xml file.

This code worked on the customer's 735xt when built with SDK 4.1.7

I suspect that SDK 4.2.x has broken something or there's now an incompatibility between the underlying code in the 735xt and SDK 4.2.x

Can anyone help me understand how to find out what that call is, and why it's now crashing on the physical watch and not on the Sim ?

Thanks in advance.