How to determine which setWatch callback function triggers the E.getErrorFlags() CALLBACK error

Try stripping your code back until you get just that error.

Normally, when something like that happens there'll be a stack trace written to the console, but it's possible you are executing code on upload, and it's that code which is causing the issue.

A new flag has appeared, 'CALLBACK'

It appeared because I imagine you modified code such that a callback function caused an error.

Although sometimes it happens if you Ctrl-C out of a function that's executing a lot in the background.