There are no changes in mod/lti which you can see by comparing stable branches in moodle repository
So error is somewhere else.
I would suggest to turn on error level to maximum in both PHP.ini and Moodle dev settings on your server and watch what appears there.
Also there are some serious errors still present in mod/lti that can break execution of LTI service.
For example in mod/lti/OAuthBody.php there are exceptions being thrown without proper namespace prefix. That immediatly cuts the execution in case of an error and you have no clue what is happening.
That is exactly what happens in handleOAuthBodyPOST function. So all throw new Exception lines should be replaced with throw new \Exception .