We are currently running Moodle 3.3 Build 20170515 with the Essential Theme (only modification is for color scheme and logo), on 4.9.95-gentoo, PHP 5.6.35, and mySQL DB 5.6.39. We had no problems with the installation last summer, and are pleased with the new theme and performance improvement.
However, sporadically, some users attempting to message a specific teacher consistently get an ""Invalid parameter value detected" error when they bring up the message entry window or try to select the teacher from the dropdown. Other users are able to send messages to this teacher. So far as we can tell, no other recipient triggers the response, although it is possible this is simply not being reported. We cannot maintain debugging status indefinitely due to space limitations, but during test periods we have not seen any error messages associated with this event.
I am not sure what to check next. This issue is impeding the ability of users to reach the teacher to confirm test proctoring using our standard secure documented processes.