This seems to be caused by the removal of the Documents query type from the query type selection in Panels. I'm not sure whether this is a bug or intentional, but I couldn't find a reference to it in the changelog.
- Add a document search query panel to a dashboard while still on week 7 stable (option has disappeared in week 8 stable).
- Upgrade to current stable branch (week 8).
- All query panels are now stuck with a 'Loading...' message instead of their actual content.
- Remove document search panel. Other query panels start working again.
phabricator c8f5fb120573f6b1a001ba78f4f8f5bc1c453e43 (Sat, Feb 25) (branched from c15501fc9b06f46a699bcded6d2e14c5c48af2db on origin)
arcanist 822bc53ca306e06314560d8a76f68771d732e8e0 (Sat, Feb 25) (branched from 3b6b523c2b236e3724a1e115f126cb6fd05fa128 on origin)
phutil e3381770af0fa8a01ca1e1078b041c00c7baab5a (Sat, Feb 25) (branched from 0625e4d28b16cba8b0154e49dc31845559b4e704 on origin)