This happened to me earlier on the current exp branch (v0.55.126981). I was in Elektro and had been playing alone on the server for about an hour as a fresh spawn with no issues. Suddenly I'm getting mauled by an unknown enemy. Figuring out that its a glitch I start wildly swinging my melee weapon in all directions thinking I can kill this invisible zed with a well placed swing but to no avail, I died.
It just happened again on another character on the same exp build only this time I was able to glean some insight from the ordeal. I ran around a building hoping to get away and found a zombie attacking dead air with suspiciously coincidental timing as the hits I was receiving. I promptly dispatched the bastard and to no surprise the attacks stopped. Perhaps this will shed some light on exactly what's going on here.