Checkout Never Advances to PayPal, Order Created in WHMCS, Cart Empty

I just about have the WP Integration working. I can place an order directly in WHMCS. I have switched to the “Cart” Order template, because that was confirmed to work in another thread.

The symptoms are as in the subject…screen never advances to Paypal, and returns to an empty screen.

Fortunately, WHMCS sends all 4 emails, one of which, if read carefully, contains a link to the invoice. So that provides a backdoor way to get this done, but far from optimal.

I don’t have an SSL cert on my MU site, am running the Ultimatum theme, and WHMCS 5.03- can’t think of anything else notable.

Actually, I did have Maintenance Mode enabled. I disabled that, but I still get bounded back to the WHMCS cart with “Your Shopping Cart is Empty”, the order is in WHMCS, but no site is provisioned, because the user never gets a chance to pay.

Maybe I missed a setting somewhere…

Ideas, please…