Checkout doesn't work

I have the exact same problem. Any working solutions would be appreciated, since it seems reverting back to the older version (including database revisions) will be a PITA.

@karmatiger enable debug in wp-config.php.
You have some errors most likely.

Have you turned debug on and enabled error logging yet?

In your header, I put just below the closing </head> tag

<script type="text/javascript">
    var wpsc_ajax = wpsc_vars.wpsc_ajax;
</script>

Will sort the wpsc_vars error for now and it works.

Also have you looked for this?

You need to run DB updates for it to work.