If you're getting the slider before the error: find something you want to upvote 100%. Drag the slider down a ways and then back to 100, then vote. After that you should be able to vote normally.

If you're not getting the slider I don't know that I can help; when it happens to me I glare at it for a while and eventually get the slider.

It's not related to yesterday's issues; @m-sargon has had this off an on for a month, and it hit my own account once.

Also when it doesn't work on Steemit I've been able to vote through Busy.