Memberships do not activate correctly

When a customer signs up to my web site on a monthly subscription they cannot immediately log in, even if the payment has been processed.

I'm using PayPal Express.

Here's a breakdown of what I know is happening:

1) User creates a new account.

2) User pays via PayPal Express.

3) User emails me for support wondering why they can't access protected content.

4) I check Membership > All Members in my WordPress plugin interface and see that that the user does in fact exist, they do have the correct Subscription, they do have the correct Membership Level and it does say "Active" in the Active column.

From the interface, all looks fine, but the user still cannot access the protected content.

I've discovered that I can solve the problem by manually Deactivating the member and immediately manually Activating him again.

Obviously, this solution does not scale.

I'm wondering whether the problem is being caused by PayPal's IPN thing, which I don't fully understand.

Right now, my PayPal account has IPN disabled. I could enable it, but I have no idea what to put in the Notification URL field and I've been unable to find documentation on this site.

Please help, because this is affecting every single new user and it's just getting embarrassing.