Custom control with ajax callback

Content

I have created a PHP custom control with 2 fields and a button to retrieve a LOV from SvC and add it to a dropdown on screen. The data retrieval is via an ajax callback to another PHP class using GET. When tested standalone, the ajax "success" callback option executes and the dropdown is populated and this works in both Development and Production mode. However when added to OPA as a custom control, the control displays but the ajax "error" callback is executed. The text of the error is "Internal Server Error".

Is there any security restriction on the use of ajax callbacks in OPA custom controls?