"Couldn't flush line" when trying to upload files using FTP

[reasv]

I tried serving a tape via FTP. The same tape and drive work fine using LTFS. After using mkltfs -w to unpartition it, I tried STFS with it, but I can't seem to write any files via FTP.
I'm on Ubuntu 22.04, using an HPE LTO9 standalone drive and an LTO9 tape.

❯ stfs serve ftp \
    -d /dev/nst0 \
    -m /root/stfs/metadata.sqlite
{"time":1735142752,"level":"INFO","event":"FTP server listening","data":[{"laddr":":1337"}]}
{"time":1735142752,"level":"INFO","event":"Listening...","data":["address",{"IP":"::","Port":1337,"Zone":""}]}
{"time":1735142752,"level":"INFO","event":"Starting...","data":null}


{"time":1735143629,"level":"WARN","event":"Couldn't flush line","data":["err",{"Op":"write","Net":"tcp","Source":{"IP":"192.168.1.16","Port":1337,"Zone":""},"Addr":{"IP":"192.168.1.14","Port":51023,"Zone":""},"Err":{}}]}

When using Windows explorer as FTP client to write a file, it starts the file transfer, then it fails midway, but no logs are generated as a result, basically I got up to {"time":1735142752,"level":"INFO","event":"Starting...","data":null} when the FTP server started and nothing else, it just fails on the FTP client side.

Using filezilla, it produces the error logs above:

{"time":1735143629,"level":"WARN","event":"Couldn't flush line","data":["err",{"Op":"write","Net":"tcp","Source":{"IP":"192.168.1.10","Port":1337,"Zone":""},"Addr":{"IP":"192.168.1.11","Port":51023,"Zone":""},"Err":{}}]}

It also seems like a 0-byte file is created in the latter case, using filezilla, though the file is obviously useless.

I should mention that, regardless of what client I used, the tape very much did spring into action and make a lot of noise like it was writing, and for a while, it looks like the transfer is working. It just fails midway through.