Terrain collision failing - reasons?

I find I typically have 4-6 instances of my game running, they run fine, then seemingly randomly one instance will fail to allow the player capsule to collide with the terrain (in the latest case this happened after the instance had been running for 9 hours). Once this has happened, any player that joins the server falls forever, this continues (for hours) until I force a restart by re-publishing the game.

My first thought was that this was due to an uncontrolled build up of network objects due to spawning without destruction, so I added some extra code to look for and destroy these objects should there be too many of them. This did not solve my problem.

I feel that this must be a server-side issue. I can see it affecting many clients, regardless of which or how many are connected. But my server loads should be quite light. Any thoughts would be welcome as I am out of ideas at the moment. I enabled logging and I hope to be able to log into a failed instance and see if there are any obvious problems, but I have not had the chance to do so yet.

I'm fairly certain I have found the answer to this - I had too many physics objects. Although the total network count was OK, it appears that moving physics objects that are in collision generate a lot of network traffic and I was exceeding the limit. Since removing them the problem seems to have gone away.