Do you see any logs on the TFTP server when you try to download the code?
I saw a similar issue where even though there were no connectivity or configuration issues, the TFTP server was refusing the connection for no apparent reason. The logs on the server showed the firewall trying to access the file, but the download failed. I ended up using a different server software on the same host (actually I switched to FTP), and the file downloaded just fine.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...