I think this bug is a release critical, so just for the record:

1) There is no documentation about stopwatches in the manual,
i.e. how to define, use them and what are consequences.

2) safety verification suddenly switches to "probably" and does not give any
trace, although the verification terminates quite quickly, i.e. it clearly
found something, but refuses to show it. 

I'd like to get that trace, even if that is an overapproximation, or at least
show a reason in status why there is no trace when trace is requested.

Is it a fundamental/correctness problem or implementation limitation?

Fixed in rev. 4247.