Test as admin. Ensure web services are enabled on Advanced features / Enable web services, and the REST protocol is enabled on Plugins / Web services / Manage protocols Go to Plugins / Web services / External services; click Add at bottom Fill in form options: 'Stupid name', 'stupid', enabled; submit. Add function core_fetch_notifications. Go to Plugins / Web services / Manage tokens; click Add at bottom. Type 'admin' username, Select the stupid service; submit. Find the right token, copy and paste it into this URL: https://YOUR.MOODLE.ROOT/webservice/rest/server.php?moodlewsrestformat=json&wsfunction=core_fetch_notifications&contextid=1&wstoken=YOURTOKEN The URL should work and return [] (empty JSON array), assuming you don't have any notifications Edit code lib/external/externallib.php, function fetch_notifications. Insert this line at start of function: throw new \Exception('Some other exception'); Reload the URL above. EXPECTED: You should get a JSON object containing 'Some other exception' text and other details. It does not have an error code. BEFORE FIX: Depending on platform or PHP configuration, you either get an approximately correct JSON with errorcode set to 'null', or else you get a blank result with an HTTP status error. In the latter case, if you look in your Apache error log you will find an error like this: [Wed Feb 27 10:25:38.825009 2019] [php7:error] [pid 836:tid 968] [client 127.0.0.1:54012] PHP Fatal error: Uncaught Exception: Notice: Undefined property: Exception::$errorcode in (system path) webservice rest locallib.php on line 154 [...]