I was able to reproduce this once or twice in IE11 here. My reproduction steps were:
Go to New Posts (probably not relevant, I was just trying to reproduce it as described)
Click Mark Forums Read to load the overlay
Click Mark Forums Read to submit that.
I must have tried it 50 - 100 times and it only happened twice. When it happened, it's like the request is aborted almost the instant the Submit button is clicked. It doesn't matter whether there are New Posts or not.
I would be specifically interested to hear if the same behaviour can be reproduced in another browser.
Unfortunately, I don't see what we can do about this. There's nothing special about this overlay. I have seen the network trace where it happened but I didn't see anything special or identifying. It seems like a strange browser quirk.