WHMCS 5.3.3; Error "Invalid filename for redirect: id=xxx" when upgrade/downgrade config optinos

Hello all,

I just updated to WHMCS 5.3.3 from 5.2.15 and I'm getting this error "Invalid filename for redirect: id=xxx" when my clients upgrade/downgrade configuration options after payment method was chosen and "Click to continue" button is clicked. Both "PayPal" or "Offline Credit Card" have same problem.

When the problem occured, URL display is "http://www.mysite.com/upgrade.php?step=4".

Please help. It is urgent.

Or is there a way I can downgrade whmcs to previous release (5.2.15).

Thanks in advanced!