Just checked out the Private Messaging feature. To reproduce:
1. I selected to Create a new PM. 2. Selected All Members option. 3. Changed my mind and decided to only send Huwr a message, so I unchecked the All Members option. 4. Clicked in the TO field but the name 'All Members' (in red) in the TO field couldn't be removed. Backspace or delete didn't remove the All Members in the field. 5. So I clicked to go back to the inbox and then create a new PM and entered Huwr's name and it worked.
BTW, isn't that an easy way to spam members?
Using Firefox 56 on Windows 10.

only Admins/Moderators can PM all members, so shouldn't be a spam problem

should be fixed here now, error in the js file, was not resetting the fields if checkbox was unchecked

This works now. Thanks Huw.