OK, you have to do some low-level debugging.
First of all, do you know exactly what operating system your *server* is running?
Secondly, do you have access to your server’s log files? Specificly the Apache error_log file? (The web server software is Apache, right?)
The next step is to see what the Amazon AJAX php code is doing. In the location field of your web browser type this:
https://yourhost.yourdomain/wp-content/plugins/WebLibrarian/AWSXmlGet.php?Operation=ItemSearch&ResponseGroup=Small&SearchIndex=Books&Title=Roadside%20Picnic&ItemPage=1&nocache=148371.90017427094
Replace ‘yourhost.yourdomain’ with your actual server host name. This *should* result in either a proper XML response or an error message (in your case probably an error message of some sort). The error message will probably not be very informative and we will need to look at the server error log to find the problem. Somewhere near the end of the error log will be reference to AWSXmlGet.php and some PHP error messages. These error messages will tell us what is wrong. I suspect there is some server config error somewhere — *I* am not able to duplicate this error so I can’t debug it on my server — we have to debug it on your server.
Note: if you get a proper XML response, then the problem is (somehow?) with your web browser.