If I was under some Slowloris attack or if some other issue would case that connections can not be accepted any more or threads would be exhausted, wouldn't this be logged into apache log?
Probably not since there was no connection being established to error in the first place and you are not running out of resources which would show up.
My personal suggestion would be similar to the same Steffen gave you over at the apache lounge (but only the AcceptFilter lines), not that it's "normal" but it is "quite common".
I think there is an way to test this however, when Apache is doing this, get into the server and try connecting via
http://127.0.0.1 (not localhost), if it accepts the connection then it is truly the AcceptFilter issue even if you are not seeing that error. Using the lan IP for the server from another computer on the lan may also be possible even though the server is not responding to the normal way of contacting it (via domain name).
Regardless, it's worth a try if only to confirm this is not the problem.