In lib/ltiprovider/src/OAuth/OAuthRequest.php a request object is created to verify the OAuth signature for accessing content on moodle.
The server name is received from $_SERVER['SERVER_NAME'].
This method for determining the server name fails if the front end name of the moodle server is not the same as the backend name which might be true if you use proxy server e.g. for security purposes.
I propose to use the configuration variable $CFG->wwwroot or the Request Url as used in many other places to determine the frontend name of the server.