While browsing the English Wikipedia today, I had several cross-wiki Echo notifications of messages that had been left on my talk page on other projects in the past. Most of these notifications worked fine, but the one for the Italian Wikisource gave the following error message:
Could not retrieve notifications. Please try again. (Error Access to the remote domain was denied.)
This message appears when you click on the Echo message icon, then click on the "View 1 message" dropdown on the "More messages from another wiki" notification.
The notification was removed after I visited my talk page on the Italian Wikisource manually (by entering "wikisource:it:" in the search bar and then clicking on the "discussioni" link at the top), so that part is working fine.
I was able to replicate this behaviour by logging into a different account and leaving another message on my itwikisource talk page. I also got the same error from leaving a message on my dewikisource talk page, so it appears that all language versions of Wikisource are affected.