Hi,
The URL is provided by the PayPal payment plugin to PayPal automatically, so normally, the URL is always correct and there is no need to change it.
The URL you provided likely indicate that PayPal is not using the URL provided by the plugin but another one (maybe one set in your merchant account ?). I think the issue is related to this.
Note that I can also see that you're using the old PayPal plugin. I would recommend switching to "PayPal Checkout". It's the newer plugin which works better, has less potential issues, and more capabilities than the old plugin. You won't have the issue with the new plugin.