Unity locking up with 0% CPU Usage

Hey All,

Our team has been having an issue lately where the editor will lock up and become unresponsive. When it gets into this state, the editor is in play mode, doesn't respond to any input (including moving the window around or trying to close it), and must be force closed via the task manager. Windows never reports the editor as "unresponsive" and it's using <1% CPU. Attaching visual studio and trying to pause it results in visual studio also locking up in a similar way. This is happening on multiple machines.

How it happens:
- We start our application running in a default scene that has a login screen
- We click a button to connect to our server, which causes 1 or 2 other scenes to start loading asynchronously with LoadSceneMode.Additive
- Unity locks up while the scenes are loading. In the hierarchy we can see the scenes are in the process of loading, but haven't finished yet

What we've tried:
- Removing all objects from the scene files being loaded, except for one collider for the player to stand on
- This issue started in 5.3.3, and we've updated to 5.3.5
- Removing all temporary directories (including library, etc) and pulling a clean copy of the project from our repo

Because it's hit-or-miss, the only "fix" we've been able to find is to keep trying to play the game over and over until it starts working again.

Any advice on how we could track down and fix this issue is appreciated.

 

Does this just happen in the editor? How does it behave in a standalone build?

 

We haven't experienced any issues with standalone builds - which we make nightly builds which then go through a QA processes.