I ran into this and found it rather unfriendly, when I wanted to refresh the mod_cluster manager (mcm) server returned Error 500. This happens when the last command was operating with a node which is no longer in the list. For example disabling a context:

http://jawa11:7777/mod_cluster-manager?nonce=a6302da7-faa0-4338-8d35-9a254f3c1e1c&Cmd=DISABLE-APP&Range=CONTEXT&JVMRoute=node3&Alias=localhost&Context=/invoker

You can easily run into this since there is no clear way - a link - to get to the page without any parameters (supposingly following the Autorefresh link which is not always desired to turn it off).

Thanks for considering!