Hello,
Most likely the reason for this error is location of file response.php
Assuming you've placed that file in
modules/CRM/Contacts/
You should use the following line instead:
eval_js("new Ajax.Autocompleter('search','update','modules/CRM/Contacts/response.php, {} );");
Also, please keep in ind that introducing changes directly to files that are part of epesi will make upgrade procedures very difficult. There are mechanisms that allow you to manipulate existing parts of the program, without direct edit of source files. For instance, to specify different tpl (like one in your own module) for Contact page you can use the following line in your custom module installation method:
Utils_RecordBrowserCommon::set_tpl('contact',Base_ThemeCommon::get_template_filename('Custom/MyModule', 'Contact'));
Additionally, since 1.0 epesi will support autocomplete as QucikForm element type.
Hope this helps,
Arek