jQuery 1.5.1 breaks all ajax() calls

Question

When I upgrade to jQuery 1.5.1 (or 1.5) all of the ajax() calls in my site produce a "parserror" in the error option function. There is also a script error

Uncaught SyntaxError: Unexpected token : jquery-1.5.1.min.js:16

The site has been running w/o errors using 1.4.4. Here is code from one of the ajax() calls.

$.ajax({
  url: '/CustomerGroup/Get',
  type: 'POST',
  contentType: 'application/json; charset=utf-8',
  dataType: 'json',
  success: function (grp) {
    if (grp != null) {
      clear();
      group = grp;
      load(grp);
    } else{
        showError(
                    'Customer Group',
                    'Whoops, error getting customer group information. Please contact support@myorg.com and include your username and date/time of the error.'
                    );
            }
  },
  error: function (x,s,e) {
    showError(
      'Customer Group',
      'Whoops, error getting customer group information. Please contact support@myorg.com and include your username and date/time of the error.'
      );
  }
});

After much research I can not figure out why the error is occurring. Any insights appreciated.

EDITED: With the full version of jQuery I get the following:

Uncaught SyntaxError: Unexpected token :
d.d.extend.globalEvaljquery-1.5.1.js:16
d.ajaxSetup.converters.text scriptjquery-1.5.1.js:16
bJjquery-1.5.1.js:16
wjquery-1.5.1.js:16
d.support.ajax.d.ajaxTransport.send.cjquery-1.5.1.js:16

and YES I am using jquery.validate.

Answer 1

This is a bug in the jQuery validation plugin. I hit the exact same problem two days ago. As it says on the jQuery validation plugin site, version 1.7 is not compatible with jQuery 1.5.x.

You need to install the newer version of validate from Jörn's github page.

Answer 2

see http://bugs.jquery.com/ticket/8302

and try dataType: 'text json' in your code