It's running 3.2.7 (by taking a peek at your changelog
Everything looks fine with the login form on your style, and the cookie settings look fine to me too. But I get "The submitted form was invalid. Try submitting again."
when I try to login on your site, too. So "it's not just you", nor a need to delete your local cookies or similar in your local browser or similar.
For sites that have a real persistent issue with this, turning off the "Tie forms to guest sessions"
configuration in phpBB can help undo part of a change that was made in phpBB 3.2.6 and later
When you can't get into ACP, Steve posted an SQL update you can run through myPhpAdmin
or similar to update the configuration database directly, using your hosting account access instead of ACP.
I don't know why your site is having this issue; only that the possibility of this issue was introduced in phpBB 3.2.6 and later. But setting "Tie forms to guest sessions"
may help work around the issue for now.