This is a long running issue for several years now.
The usual suggestion would be to clear the cache and cookies of your browser and check that cookies are allowed for a sub set of sites, like pioneerdj, rekordbox, zendesk, gigya.
Sadly that doesn't work for some.
The steps I follow when arriving on the forum are to login (or log out if it shows me logged in). Wait for the page to refresh. Log in and I'm good to go.