Other sites mostly working? Then your MTU setting is incorrect.
Try:

ip6tables -t mangle -A FORWARD -p tcp –tcp-flags SYN,RST SYN -j TCPMSS –set-mss 1452

And in radvd.conf (interface xxx section): AdvLinkMTU 1452;

Renew your Windows clients with ipconfig /renew6 and then check MTU with netsh interface ipv6 show subinterfaces, it should be 1452. You can test whether you have an MTU issue by visiting http://test-ipv6.com (WARNING: website doesn’t always work reliably, if it shows errors, visit http://test-ipv6.ch to confirm it’s real) – it will do a large packet test and if it fails, your system is not fragmenting packets correctly.