We are using PayPal express payments and the OneStepCheckout module.
A customer reported that he tried to make an order, but when clicking "buy" the waiting animation keeps being displayed and he is never be forwarded to the PayPal website.
When we tested this, we could not reproduce the problem.
After cleaning the Magento caches, the same customer was able to place an order (while it is not clear if this is related).
What can be the reasons for this? How can the problem be troubleshooted?
Ideally it would be easier to troubleshoot if you were able to reproduce the problem but since that isn't the case your best chance would be (IMO) to just add lots of logging in the module.
Mage::log('Customer did this')
Mage::log('Customer did that')
Mage::log('Bleh')
Mage::log('Meh');
Afterward if the problem might occur again you would have something to build on.
Continuous animation means that AJAX call never returned or returned something totally wrong. Either check your PHP log for Fatal Errors or Magento exception.log, however any decent module would be prepared to handle ordinary exception, so my bet is on a Fatal error.
If the icons are spinning then it usually means issues with ajax requests and you can easily observe what is wrong with them or what they are responding to you with developer tools provided with every modern browser.
Your question does not provide essential info (browser version etc) for solving this without overviewing your site. There can be php, javascript or caching issues on your site. They all are rather easy to debug with logging, and developer tools
In case of paid extensions I suggest you to contact support e-mail as support is usually free and usually the issues are site/theme specific.
Does your server have a PHP error log you could check? any error would be returned there. Otherwise you could use firebug or chrome developer tool to see the Ajax response. Also turn on
ini_set('display_errors', 1);
in the index.php to make sure all errors are outputted
