Thymeleaf - How to compare string with request parameter in html in Thymeleaf tag "th:if"?

It's not working because param.error is array of strings. You must retrieve first element of array (param.error[0]) to get first value of parameter (see documentation). Besides that you can access request parameter via Web context object method #httpServletRequest.getParameter that returns first value when parameter is multivalued (see documentation).

1. Usage of Web context namespaces for request attributes

   <div class="error" th:if="${param.error[0] == 'badCredentialsException'}" th:with="errorMsg=#{login.badCredentials}">                      
       <p class="errorMsg"><span th:text="${errorMsg}"></span></p>
   </div>
   

2. Usage of Web context object

   <div class="error" th:if="${#httpServletRequest.getParameter('error') == 'badCredentialsException'}" th:with="errorMsg=#{login.badCredentials}">                      
       <p class="errorMsg"><span th:text="${errorMsg}"></span></p>
   </div>
   

With Thymeleaf 3, I normally use #request (a short form of #httpservletrequest) and #strings.equals() for that, which will look like this:

<div th:if="${#strings.equals(#request.getParameter('error'), 'badCredentialsException')}"></div>