Incorrect Resource URL error when they're correct
We deduced that this error message comes from the timeout being too small in the public function  fileExists($url, $timeout = 0.50) in civicrm-core/CRM/Utils/Check/Component.php.

The timeout is not long enough for all sites which is why the message pops up despite the settings being correct.

This can be resolved by either hacking the source code and increasing the timeout to something bigger, for example 1.0 or turning off the check.

You can see the gitlab issue here for more information regarding this: https://lab.civicrm.org/dev/core/issues/1035

I believe that the Civi team are working on a fix for this as well to avoid having to hack the source code.