In addition to archil's answer, don't forget that if you require authentication, the allowed users and roles are case sensitive:
<appSettings>
...
<add key="elmah.mvc.disableHandler" value="false"/>
<add key="elmah.mvc.disableHandleErrorFilter" value="false"/>
<add key="elmah.mvc.requiresAuthentication" value="true"/>
<add key="elmah.mvc.IgnoreDefaultRoute" value="false"/>
<!-- The roles and usernames for elmah are case sensitive! -->
<add key="elmah.mvc.allowedRoles" value="Administrator"/>
<add key="elmah.mvc.allowedUsers" value="user1,user2,user3"/>
<add key="elmah.mvc.route" value="elmah"/>
...
</appSettings>