I modified clock.cc to add a boolean named "started". I initialized it to false in the constructor, set it to true inside the start() method, and call abort() inside stop() if "started" is false. It fails for me during every run.
This is a problem because the variable "last" inside the clock class is uninitialized, and is potentially random.
I'm not sure what's going wrong, as it appears that funcs->start() is being called inside CactusTimers.c, but somehow the relevant start function isn't called in clock.cc.