I am having the same problem as werkhaus was having.
I contacted my host and it is not a sessions error.
They fixed the sessions error and now my simple products add to cart at least.
I checked all my plugins and it is not an interference error.
when i add a variable product it just get a prompt saying please choose product options…
My site is http://www.thrillseekercollective.com for reference.
is there anything else it could be? Thanks