Same here:

First time I died with "My leg is in pain", "I think my leg is broken" and "You are dead" when I was running fast uphill northwest of Berezino. No gunshots and no glitching, just instant dead.

Second time was when running over the nwaf between the landing strips, no other players on the server, i checked really every 10 seconds. Again, no shots, just while running fast I'm dead, leg broken.

Third time was in the storage barn at the nwaf, again no other player on the server and protected by walls, I logged in within the storage barn, started running fast and instantly died after two meters.

Fourth time I logged in in a bush nw of Berezino. I didn't move one bit, just turned around, looked at my rear and turned around again to the front. And died there in the bush. No other player at the server.

Conclusion:

It happens at different locations, mostly while running fast, sometimes while standing still, but almost every time the height of the land is the same, so I'm definitivly sure, it has nothing to do with lagging and the server glitching me to a position, where I could fall down. Also, I never got shot, there were no other players on the server, no zombies near me nor other reasons that could have broken my leg.

I have no problems with running in buildings on upper floors. Never died up there, never got ruined items or broken legs.