Recently the GPG signatures on one of the upstream repositories expired before it could be renewed. A fix should have propagated to all mirrors by now, and may require action on your part to correct.
For unknown reasons, the correct solution appears to vary somewhat by user, so multiple methods are given here that have proved to be successful:
- Using Synaptic:
a. Reload source lists. You may receive an error message.
b. Mark all upgrades and apply.
c. When system pauses and asks whether to accept maintainers configuration changes, click accept.