Page Folders don't load

[Kagetsuki]

Folders stopped loading reliably after a point (we have almost 300 folders/sub folders). I think it's some sort of timeout issue while building the collection, but there are JS load errors as well. This only started occurring as we added more folders. I'm not seeing anything in any of my logs, and the DB connections aren't timing out or anything, so I actually don't have any information other than what I'm getting in the browser. Screen shot for reference:

If there's some extra information I can provide please tell me what and how to get it.
I've confirmed this is occurring in 6.3.6 and as a test tried a 6.2 release - both have the same issue.

[yuji]

@Kagetsuki

Hello,
Can you check http request log in Network tab? Folder data is loading by Ajax request. I guess that request is timed out. If so, please extend the request timeout setting of the web server, then try again.

[Kagetsuki]

Network tab shows no timeouts, only a 404 on mt_ja.js:

Realizing the JA locale may have been an issue, I changed to EN, and the 404 stopped (no failures in network) but the folders still don't load:

The error message in the console appears the same in both JA and EN: