jQuery 1.5.1 (or 1.5) breaks all ajax() calls

jQuery 1.5.1 (or 1.5) breaks all ajax() calls

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". 

$.ajax({
  url
: '/CustomerGroup/Get',
  type
: 'POST',
  contentType
: 'application/json; charset=utf-8',
  dataType
: 'json',
  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.'
      
);
  
}
});