Make sure that the file: modules/mod_ajaxscroller/mod_ajaxscroller.php has execution rights (CHMOD 644). If directories modules/mod_ajaxscroller/ and modules/ are CHMOD 777 change them to 755.
If you use special .htaccess rules for extra security make sure you exclude module/mod_ajaxscroller/ to allow script execution from this directory.
Try to access the file directly through your browser (http://www.domain.com/modules/mod_ajaxscroller/mod_ajaxscroller.php?m=1&d=next&c=0). You should get a text response and not Error 500 nor Error 403.
Another reason why the module doesn't scroll can be a JS error. Check that you don't have JS errors in your browser. A common cause of the error is a conflict with jQuery. If your site uses this JS library it may collide with Mootools, Joomla native JS library of choice which is also used by AJAX Scroller. In that case make sure that the jQuery is implemented in no-conflict mode by modules that use it. Another reason for JS error can be double inclusion of Mootools. If Mootools is already included on your site (in most cases it is), do not choose option to include it in AJAX Scroller parameters. Lastly, use tools like Firebug for Firefox to ensure your site is free of javascript errors.
If you are confused just email us the URL where you installed the module and we will quickly diagnose the problem.