When I view an issue directly (from a URL like https://mycompany.atlassian.net/browse/ISSUE-nnn), the Log Work button in the Tempo section is disabled. If I make a change to the issue, such as starting or stopping progress, the button becomes enabled. This behavior just began yesterday (2016-02-16) as far as I can tell.
I can still log time to jobs in other ways, so this isn't a show-stopper, but if I'm viewing an issue directly, that button is my go-to for logging time, and I'd like it to work properly again.