I have encountered the same problem a few times already with different guns / scopes.
Played with a Mosin w/ PU Scope yesterday and the zeroing worked just fine, logged back in today to find out it was broken (zeroing works but shows up as a black box in the HUD). I still haven't figured out what factor(s) causes it to break but my workaround for now is just to turn the zeroing all the way down and increase it in steps while counting.

You can still do that through steam, when you open the steam overlay there should be a list called "Friends on this server". You can also view a list of all the players on the server by clicking on "Current game" in the steam overlay.

Found another one in front of Vybor police station: