"An unknown error occured" (or HTTP statuscode 413)

[waweic]

In recently helped a friend installing Nextcloud AIO in a VM on Proxmox, with a reverse proxy (Nginx) in a different VM and the NEXTCLOUD_DATADIR env variable set to a mounted BTRFS subvolume.

When uploading any file larger than some MB, Nextcloud displays the error message "An unknown error occured". In the Firefox Network log, we can see a 413 response, from Apache, if I recall correctly.

Has anybody seen this before? Any ideas on what we could try? I figured that even tough it's probably a mistake in our configuration, it may not be a bad idea to have a solution to this in the documentation somewhere, if it's a common problem