Everything I can see describes "connects but won't download" as a "classic MTU problem".
I've set every MTU on my system to 1280. Here are some diagnostics...
ajy@seth:~ $ sudo ping6 ipv6now.com.au -M do -c 1 -s 1232
PING ipv6now.com.au(2600:3c01::f03c:91ff:fe93:48f8 (2600:3c01::f03c:91ff:fe93:48f8)) 1232 data bytes
1240 bytes from 2600:3c01::f03c:91ff:fe93:48f8 (2600:3c01::f03c:91ff:fe93:48f8): icmp_seq=1 ttl=57 time=142 ms
--- ipv6now.com.au ping statistics ---
1 packets transmitted, 1 received, 0% packet loss, time 0ms
rtt min/avg/max/mdev = 142.110/142.110/142.110/0.000 ms
ajy@seth:~ $ sudo ping6 ipv6now.com.au -M do -c 1 -s 1233
PING ipv6now.com.au(2600:3c01::f03c:91ff:fe93:48f8 (2600:3c01::f03c:91ff:fe93:48f8)) 1233 data bytes
ping: sendmsg: Message too long
--- ipv6now.com.au ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms
ajy@seth:~ $ sudo ping6 -c 200 -s 2000 -f ipv6now.com.au
PING ipv6now.com.au(2600:3c01::f03c:91ff:fe93:48f8) 2000 data bytes
--- ipv6now.com.au ping statistics ---
200 packets transmitted, 200 received, 0% packet loss, time 196ms
rtt min/avg/max/mdev = 143.026/143.847/151.284/1.251 ms, pipe 10, ipg/ewma 16.047/143.595 ms
ajy@seth:~ $
Do these imply anything? (other than, perhaps, that it is a TCP problem and not specifically an MTU problem)
Andy