I'm experiencing this on both MSE and SO now. I had 3 different tabs open where I was voting to close 2 posts in 2 tabs and flagging a post as spam in the third. Upon clicking the close/flag buttons, all the 3 flag/close pop-up windows seemed to load indefinitely without disappearing.
The close votes and the flag votes are also not being taken in by them.
Can someone explain what exactly is happening here, because this is something I'm experiencing for quite a while now (20-30 mins). Here is a sample snapshot.
Update: As Paul mentioned, it seems to be a bug and I confirmed it after inspecting the console when I clicked on the button. Here is a snapshot of the error.
On further inspection, I could see that there is another error which is possibly related to this. On the page load of any post, there is an error which is thrown in the console. Here is a snapshot of that error.