Thanks for the comments. Like @simpson_jr I have to log out to be able to access the reference stuff.
I have similar problem (reported over a year ago now) that logged onto forum if I open a tab in the main site then I get logged out. Really annoying when your half way through replying to someone and just checking something on the reference pages get you logged out and reply deleted.
SSO is all the rage on families of related web sites (e.g. your bank, their brokerage group, their insurance group, their mortgage arm) and Arduino seems to have got the bug and bungled the implementation.
Why they would ever want to implement such a thing, beggars the imagination (though someone may interject that it relates to the "shopping trolley" which is the absolute least important thing here!).