I can fetch the header of websites via IPv6 but not the webpage, why?

[Ez mail]

Hi!

Since I have no fr**king clue what could the problem be, I'm trying on 
this list :)

I have a native IPv6 connection on my Desktop behind my router.

curl -v -6 http://anyurl.foo

doesn't work, the connection just times out SOMETIMES (when there is a 
problem, tcpdump on the router says I doesn't even receive any packets 
back, ip6tables is empty on the Desktop).. BUT a:

curl -v -6 --head http://anyurl.foo

is perfectly working. See my ping from my Desktop:

root at server:~# ping6 -c 10 -s 2000 -M do IPV6ADDRESSOFANYURL.FOO
PING IPV6ADDRESSOFANYURL.FOO(IPV6ADDRESSOFANYURL.FOO)
2000 data bytes From IPV6ADDRESSOFSOMEGATEWAY icmp_seq=1 Packet too big: 
mtu=1452
2008 bytes from IPV6ADDRESSOFANYURL.FOO: icmp_seq=2 ttl=54 time=82.5 ms
2008 bytes from IPV6ADDRESSOFANYURL.FOO: icmp_seq=3 ttl=54 time=82.4 ms

soo... it couldn't be an MTU problem.. since the ping is ok.. (?) -> 
there are icmp echo replies..

WHY does the connection "randomly gets bad"? (so sometimes it works, 
sometimes not, the "anyurl" is ex.: youtube, facebook, and it works from 
other places via IPv6 when it doesn't work behind my Desktop..)

Any help would be appreciated.. a keyword to what to google for or sth.

IMHO the problem isn't on my Desktop or on the remote site (other 
Desktops behind the router are sometimes bad).

Maybe between somewhere, but how can I know which GW is bad that are 
shown in a traceroute?

(xxx is my original IPv6 addres, just censored)

user at desktop:~$ traceroute6 -I k.root-servers.org
traceroute to www.ripe.net (2001:67c:2e8:22::c100:68b) from 
2001:4c48:XXXXXX, 30 hops max, 60 bytes packets
1 20014C48XXXXXX.dsl-dp.pool.telekom.hu (2001:4c48:XXXXXX) 0.546 ms 
0.397 ms 0.325 ms
2 2001:4c48:0:12::2 (2001:4c48:0:12::2) 17.799 ms 18.316 ms 18.597 ms
3 2001:4c48:0:11::1 (2001:4c48:0:11::1) 19.013 ms 19.210 ms 18.043 ms
4 2001:7f8:35::6939:1 (2001:7f8:35::6939:1) 30.447 ms 22.476 ms 25.313 ms
5 10ge1-4.core1.vie1.he.net (2001:470:0:2b3::1) 25.142 ms 25.653 ms 
25.066 ms
6 10ge1-4.core1.prg1.he.net (2001:470:0:284::2) 30.705 ms 30.731 ms 
32.774 ms
7 10ge15-3.core1.fra1.he.net (2001:470:0:213::1) 38.737 ms 51.226 ms 
54.640 ms Receive error: Permission denied
8 100ge3-1.core1.ams1.he.net (2001:470:0:2d4::1) 44.183 ms 43.524 ms 
44.071 ms Receive error: Permission denied
9 amsix-501.xe-0-0-0.jun1.bit-1.network.bit.nl (2001:7f8:1::a501:2859:2) 
45.437 ms 46.198 ms 45.206 ms Receive error: Permission denied
10 gw.ipv6.amsix.nikrtr.ripe.net (2001:7f8:1::a500:3333:1) 52.527 ms !A 
54.040 ms !A 52.016 ms !A

The "Permission denied" appears even when trying with root

Thanks!