With this plugin enabled, contacts do no longer load when composing mails. Clicking an address book in the left pane (larry skin) results in either the first or the first 3 contacts being loaded, then the "loading" overlay keeps spinning but nothing more happens.
I've checked the error console and found a clue: "rcmail.env.messages is undefined" in plugins/message_label/message_label.js line 242. Addin a single line at the start of that function fixed it for me:
rcmail.addEventListener('insertrow', function(evt) {
if (rcmail.env.messages == undefined) return;
Cheers,
Izzy.