Dedicated Server Failed to Start

Same here. Caves were fine, but after logging out and trying to log in I get the same error. My friend as well. So I think it isn't a user error but a Klei problem?

You can go into the cluster file to delete the entire caves server (make a copy somewhere else), then you clan play without caves. This way you can still play above ground, but of course you'll miss the caves  quite a few people have had this problem, I hope they fix it soon. Then you can add the caves again whenever the problem is fixed. 

It could potentially be an anti virus issue, I've read about that but in my case (and my friends) it isn't. I don't even have anti virus. So good luck!