Thanks for the info, I think that's where the problem was.
I've set up a rewrite rule to add the www. at the beginning of the domain name if it's not there.
Previously, when logging in without the www at the start, the site would set the cookie to tunasnax.com
and redirect you to the pages with the www, making it work. But going back to tunasnax.com without the www resulted in the cookie not being recognised on that domain.
So now you can still just type in tunasnax.com and it will automatically change to tunasnax.com
and everything should be fine.
Thanks for helping with the diagnosis!