Is the SE team hard at work tweaking the UI for locked questions, or is the web proxy at my job randomly blocking stuff?
This is something we're experimenting with for truly "historical" questions (that is, questions and answers which no longer benefit from any interaction). They are effectively frozen in time, a record of something that happened and still has some meaning but is no longer a living, breathing part of the site.
More to come...
It's not just you.
Voting is currently disabled completely on closed questions (maybe allowed in some cases), there's a class on voting buttons
When I tried to "show" it with Chrome inspection tool and tried to vote up, this popup appeared