Hey there,

I am really sorry for the delay of this reply as it is a public holidays for us here.

Can you try download this fix patch file from this thread https://stackideas.com/forums/serialization-of-closure-is-not-allowed-when-proceeding-to-payment#reply-446012

and see how it goes.

If the issue still persists, can you update your site FTP access at http://stackideas.com/dashboard/site so we can better have a check?