I may have had a similar problem. The communication would start with the TFTP server and then hang (I would see the dots go across the screen then stop). I tried a different TFTP server and it worked fine for me. Its free, TFTPD32.
At the risk of repeating what others are saying, I'd agree with going with the free Solarwinds. It seems to just work with Cisco, whereas others I've tried (are you listening, Pumpkin?) fails, more often than not.
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...