That sounds super odd. The “FTP (alternative)” backend is the same, it just has different defaults for relative paths, SSL mode and FTP active/passive mode. These should either fail or work, not affect transfer stability. If you do not set the flag --use-ssl or the --ftp-passive / --ftp-active options, the two should do the same, and they use the same code.
That pretty much says that something failed with the stream. For the 2.1.0.4, this could this be yet another timeout issue. If so, try perhaps --read-write-timeout=5m.
For 2.1.0.5 or 2.1.0.110 the timeouts should not cause problems.
The FTP server log does not seem to have errors? But I wonder why it sends a notice about TLS after the STOR command was sent?