Getting shipping quote stalls.

Because the SOAP request is successful, I think it's unlikely that the problem is with this module (not impossible, just not the most likely suspect at this time). The progress bar on the checkout page will spin while the uc_quote JavaScript makes an AJAX request for quotes and waits for the response back from web server. So the first thing you should do is try shipping quotes with just one Flat Rate enabled (flat rate methods are computed locally on the web server and do not connect to another host to obtain a quote, like FedEx does). If the flat rate exhibits the same behavior, you know you have bigger problems. Likewise, try it with JUST FedEx enabled, no other methods. The progress bar will spin until *all* the quote methods have finished, so if one of them is holding things up then you will never get a response, even if FedEx succeeds in getting a quote.

I think the most likely problem is a JavaScript problem - things like jQuery Update occasionally interfere with the proper execution of the uc_quote and other JavaScript on the checkout page. Use the Firebug console on the checkout page to examine the AJAX request/response and to look for JavaScript errors on the page. I can take a look at the checkout page for you if you PM me a link to your site.

Perhaps a cache issue then...

So can this issue be marked as fixed/closed?

Try changing this:

        // ACCOUNT quotes return only ACCOUNT rates
        $ratedetail = $options->RatedShipmentDetails[0];

To this:

        // ACCOUNT quotes may return either ACCOUNT rates only OR
        // ACCOUNT rates and LIST rates.  Check.
        if (is_array($options->RatedShipmentDetails)) {
          $ratedetail = $options->RatedShipmentDetails[0];
        }
        else {
          $ratedetail = $options->RatedShipmentDetails;
        }

Fixed in 6.x-1.x-dev. 6.x-2.x-dev has always had this bit of code.