Steps to reproduce:
1. Create new group. For example TestGroup.
2. Add some users to new created group.
3. Create and save new filter. For example TestFilter. Filter settings:

• Project: All Projects,
• Assignee: Specify Gropu: TestGroup.
  4. Add to dashboard Filter Statistics new portlet. Filter Statistics portlet settings:
• Filter: TestFilter,
• Statistic Type: Assignee,
  5. Click on some user listed. You have HTTP Status 404.

Look at the link URL:
http://host/jira/secure/${urlPrefix}&assigneeSelect=specificuser&assignee=testuser

It constains: ${urlPrefix}
It should be replaced with something like: IssueNavigator.jspa?reset=true ...