I think it is something to do with the way headers are passed back to moodle and are possibly being stripped off.
Some people have seen this message for other LTI services - see this post https://moodle.org/mod/forum/d....
The way I would diagnose this is by editing the moodle code to print out the http packet that you get back from TA. It will be sending correctly from TA but I think at some point the packet is being changed on its way to moodle. The best way to check this is to print it out in the packet in the service.php file on the moodle server - this is the place the grade gets sent to - http://localhost:8888/moodle29/mod/lti/service.php. Of course this means you need access to a non-production version of moodle to test. Part of the problem is recreating this problem on a server where you have diagnostic control.