Error: Symbol Not Found Error / initialize() at Rez:54

Hi,

My program is running in simulator for Edge 830 and Forerunner 945 OK. But when I try and switch to vivoactive 3....

Error: Symbol Not Found Error
Details: Failed invoking <symbol>
Stack: 
  - initialize() at Rez:54 0x10001ec2 

One of Jims Posts seems to say one might get this when one tries to run with a device whose firmware doesn't have right support.
My app manifest says need 3.1.x; devices.xml says vivoactive 3 is 3.1.6 at FW 720. 

Any ideas?
So error is an an autogenerated file; where is that file located?
How to get a idea where the fault is?

Regards,

Owain