Bug#980119: Found the reason and the fix

[Tim Kosse]

Hello,

I managed to reproduce the issue, but only with the Debian package of 
GnuTLS, not with a manually compiled version.

With this information I quickly found that the problem is the patch 
48_0001-Fix-non-empty-session-id-TLS13_APPENDIX_D4.patch, it breaks TLS 
session resumption if not using TLS 1.3.

FTP over TLS uses session resumption on the data connection as a 
security measure against data connection stealing attacks.

It looks like upstream has already fixed the issue: 
https://gitlab.com/gnutls/gnutls/-/commit/05ee0d49fe93d8812ef220c7b830c4b3553ac4fd

With this additional patch applied on top of the problematic one, the 
error disappears.

Regards,
Tim Kosse