There is lots of websites that are hosted at locations that have administrators who think they know better and who thus block ICMPv6 and thus that break Path MTU discovery. For Citrix this has been the case for several years already, attempts to clueing them in has not seem to have any result. A 'tracepath6' will easily demonstrate that something there is filtering ICMP and thus also ICMPv6 Packet Too Big that is needed for path MTU discovery to work. Little to nothing SixXS can do about, this is all on the admin of the remote site.