rclone: FTP upload and download for big files (over 10GB) fails

Output of rclone version

rclone: Version “v1.55.1”

Describe the issue

This happens only for big files (over 10GB): the upload goes to 100% then rclone disconnects, reconnects to check the file on destination but declares that the upload has failed and restarts it! Adding --no-check-dest --low-level-retries 1 does not resolve the issue.

It works when uploading manually with the ftp command! So the issue is NOT with the FTP server!

Typical rclone log/verbose going into a loop:

2021-05-13 18:34:22 DEBUG : big.zip: Need to transfer - File not found at Destination
2021-05-13 18:44:53 DEBUG : ftp://server.com:21/: Connecting to FTP server
2021-05-13 18:44:54 DEBUG : big.zip: Removed after failed upload: read tcp IP1:37706->IP2:21: i/o timeout
2021-05-13 18:44:54 DEBUG : big.zip: Received error: update stor: read tcp IP1:37706->IP2:21: i/o timeout - low level retry 1/10

I suspect the FTP library to be behind this bug: there is no timeout issue, I can see the file going to 100% on the FTP destination before it gets deleted by rclone. TLS is not used.

My suggestions/questions to fix: a) Why does rclone disconnects + reconnects to check the file instead of using the same FTP session? b) Can’t rclone sleeps for 2-3 seconds before doing the check of the uploaded file?

=> see log with --dump bodies attached! rclone_issue_5328.txt