The second step of external tool registration just shows a loading image. Checking out the JavaScript console, I can see the following:

require.js:903 TypeError: a.indexOf is not a function(…)
check @ require.js:903enable @ require.js:1180
init @ require.js:786
(anonymous function) @ require.js:1464

Looks like it got broken after MDL-50937 got integrated.

To reproduce:

1. Expose your webserver on the internet using ngrok.

   ./ngrok http 80

2. Set your config.php's $CFG->wwwroot to the http (or https, if you're using https) URL shown in ngrok's console.
3. Go to Site administration ► Plugins ► Activity modules ► External tool ► Manage tools
4. Enter http://lti.tools/test/tp.php into the Tool URL... field in the Add tool panel.
5. Click the Add button.
6. Check out the page output.
7. Expected:
   • The loading indicator goes away and the http://lti.tools/test/tp.php page loads up.
   • There should be no JS error.
8. Actual
   • The loading indicator does not go away and there's nothing you can do.