At the beginning of the file ubercart/payment/uc_paypal/uc_paypal.module I read the following comment:
You should turn on IPN Notifications in your PayPal profile but leave the IPN URL blank. This allows the module to specify an IPN URL when it directs payments through PayPal that includes the order ID for debugging purposes.
I would like to know if that comment is still up to date because PayPal does not accept an empty field for the URL as you can see in the attached screenshot.
If that comment is not appropriate anymore I would definitively suggest to remove it.
If I understand post number 27 in the thread at http://www.ubercart.org/forum/support/524/paypal_ipn_integration_not_wor... (written by torgosPizza) correctly he also says that the url field is required by PayPal.