I have had it reported to me and also found for myself today that the ability to send a message to someone has stopped working on my site (possibly since one of the recent updates.)

If you try and send a message from a friends profile, when you click to send it you just get the error shown in the attached screenshot and it won't send it.

If you try and send via "Conversations" and use Compose, when you click to send (submit) you just go to a blank screen (and again, the message is not sent) - checking the page source shows it just contains the text "1"

This is quite a serious issue for me so any suggestions greatly appreciated.