Update: This issue is resolved in BrightAuthor versions 18.104.22.168 and later.
Twitter recently updated their API to version 1.1, which features a new endpoint authentication system for feeds. This authentication system is not fully compatible with the current version of BrightAuthor. As a result, you cannot add a Twitter feed to a new presentation, and feeds in currently active presentations are not working.
We are currently developing a fix for this issue. Since credentials are now required for accessing a Twitter feed, you will need to re-add Twitter feeds to presentations (and republish those presentations to your players) after installing the BrightAuthor version containing the fix.
The fix will be announced here on the Support Homepage once it is available. You will also be able to find it on the Software Downloads page.