I think this was a deliberate feature the devs implemented to cope with site downtime problems - it doesn't actually disable upvotes, just the tracking of what you've upvoted (since that was apparently really heavy in terms of performance). So while it might seem like you can't upvote, it could simply be that the site is not highlighting the upvote icon - you can test it out by upvoting something, seeing the number increment, and then refreshing the page - it won't be colored in, but it should still be incremented properly.