Ok…trying to respond to a poster on a thread. The little circle spinned around for a LONG time and then I got this…a new thing for me!
A literal answer to your question is that it’s a JavaScript error. But that’s not very helpful. 
Does this happen every time or was it just this once? If it’s not every time, that suggests something got into a weird state and the JavaScript code the site used got confused. Unfortunately, that’s very hard to debug.
If it happens every time, I can look to see if we have any recent changes that might have broken something.
Just this once. I never saw it before