Sync larger than 18KiB files not possible with 401 error

I have a docker(official image) installation on my home server and have been using it for years.
I don’t deal with large files, so I don’t do much with it, and I’ve been syncing tens to hundreds of MiB files with no problems.
A few days ago, a 9.3MiB ttf font file failed to upload on my Mac client (v3.10.0), and since then I’ve been unable to sync from client to server.
Here’s what I’ve figured out so far.

  • (Important) Up to 17 KiB, synchronization works. From 18KiB, synchronization fails with a 401 network error. And client app says “server replied : Not Found”
  • Files larger than 18 KiB fail to sync with error above, even with modifications.
  • On the web, even large files were uploaded. (However, even on a gigabit local network, uploading 1 MiB took tens of minutes)
  • Files already on the server are synced to the client without issue, regardless of size.
  • The problem is the same whether I’m at home or away.

Just to be on the safe side, I have all of the large file upload settings turned on, and I have the relevant settings turned on in the nginx proxy manager.

I’m also using Cloudflare, which I’ve been using for a few years now, so I don’t think it’s a problem, but I set the max chunk size to 100MiB just in case.

Spent whole two days with this problem.
This is a very painful situation because it was a useful tool to me.
Any help is welcome.