(Updating this thread for future reference)
Reviewing the Open Source code we found that it seems we have an incomplete commit for the PayPal functionality. The pop up is indeed commented out because it's no longer used, however it's missing the code to package up and pass this information to PayPal to process this transaction. We are working on this and should be able to get this into the Open Source branch sometime this week.
Also, if you haven't already, check that the environment variables related to donations are to ensure they are set correctly.