I just had this problem on a Drupal installation and found a cause. In my situation, it was a false positive.
To check if the resource URL is correctly set CiviCRM tries to download the following picture from the server
It does the check, however, on the server itself, ...
I am on wp and the error I have (which is just a notice) is make the resource urls are portable. That's the false positive.
I don't think the incorrect resource url error that you have is a false positive seems to be something wrong.
The alert is incorrect. As long as you are not having any other issues besides the alert, you can tell the alert not to remind you again.
In addition (now that I see the issue with Contribution pages - sorry I missed that), make sure you have the base page set under CMS settings in the CiviCRM administration settings.
In WP go to settings, Permalinks. ...
Normally it's [civicrm.root]/.
Is that screenshot just an example of values you've been playing with? Does it work with [civicrm.root]/?
And [civicrm.root]/ is what it is set to on https://wpmaster.demo.civicrm.org.
Also check your civicrm.settings.php file to see if it's overriding it for some reason.