just to clarify, it's when I click on 'add'; the button is animated for a while, then the error comes

I can't reproduce this.

I looked at the error log, and my best guess is that the cache got confused about a cardtype on a set card. I know that probably doesn't mean much to most of us, but basically things appear to be back to normal, but the frustrating thing is that it's really hard to figure out why the cache gets confused on little things like this.

if you do see this again, it's worth trying /admin/clear_cache in a separate tab to see if that fixes things.