I've experienced Editor freezes a few times, I think some of the reasons were:
1. Scene contains an object with a TNObject with no ID set. These are meant to be instantiated at runtime.
2. Malformed packets. I've only experienced this when writing custom packets. I don't remember what the exact problem was or how I fixed it.
3. Entering playmode with a scene that expects TNManager to already be present and connected. This was a problem with my own code.
This was all in TNet 2, no idea if these issues apply to TNet 3, but it's something to look at at least.
I could also see a potential problem with RFCs being executed before the server instance fully initializes? Too lazy to check the code, but also something for you to look at.