This pertains to those who have been having problems using Cloudflare’s CDN with Nextcloud and the desktop sync client when uploading large files. This has been an issue ever since the release of desktop client ver 3.4. Tracked here: "Connection closed" message when syncing files larger then +- 100Mb · Issue #4278 · nextcloud/desktop · GitHub
It seems like the newer desktop clients have not been chunking correctly. Meaning, uploading large files (>100MB) hit Cloudflare’s upload limit which causes the upload to fail.
That being said, we have found a fix! Manually overriding the chunking size in the desktop configuration file leads to proper chunking and no more errors with large files.