The same message appeared here and there and in both cases went away following actions that did not touch Apache or certificate configuration.
That makes me think that it might not be a problem of Apache configuration, rather that something else is interfering with Apache (and the “couln’t grab mutex” may be a symptom of that?). I did not understand the exact role of the timer, you could try to put it to a longer value. Or you could try to remove a package (rather a “big” one).