Ajax http 500错误

New to StackExchange, although I have read a lot on here. My problem: an Ajax error.

HTTP Result Code: 500
Path /system/ajax
no status text or response text.

This just started on a website that was working fine. I have tried increasing PHP's memory, but that isn't working either. The error report says this:

"Notice: Undefined property: stdClass::$data in uc_ups_quote()"
(line 726 of /my/path)

And:

"Exception: String could not be parsed as XML in SimpleXMLElement->__construct()"
(line 726 of same/path)

Line 726 of that file is: $response = new SimpleXMLElement($resp->data);

Any help is appreciated. I couldn't find the answer anywhere else.