After upgrading to Magento CE 126.96.36.199, when I enable Form Key Validation on Checkout as recommended by SUPEE-9767 onestep checkout stops working.
After selecting a payment method, when I click Continue Payment the one step process does not continue. I do not see any server response or logged error messages.
When I disable Form Key Validation on Checkout, the checkout process functions as expected.
This is a default Magento installation with no extended theme.
This seems to be associated with SUPEE-9767 Patch/CE 188.8.131.52 - One Page Checkout - Customer Registration issue where the ajax savepayment method call is returning null even though template files are patched and formkeys are present,
I could not replicate this on a clean 1933 install.
Here is the cause of my problem
<input name="form_key" type="hidden" value="SLLU234jmligBYb9" disabled="" autocomplete="off">
CSS is adding the disabled selector to the form_key. This means the form_key value is not submitted to the one step checkout controller causing the controller to return null. To test this, remove the disabled selector in your browser inspector.
This problem was caused by Paypal Plus which dynamically replaces the contents of fieldset at the onestep checkout payment method. See my answer below.