Hm, I would guess that's something to do with the overlay...

Does the dashboard work if you open this URL in a standalone browser (Firefox, Chrome, etc):
http://localhost:28053

Can you give any more info about the error? When does it happen? Does it happen every time or inconsistent?

What operating system are you on?

I'm using Windows and Firefox.

It only works for me when using the localhost address, which is fine. I'm tinkering with the statistics. :)

The error only happens with the in game browser, which happens every time. The in game browser doesn't work for anything in my game.

Ok, glad it works in the external browser. Given that, and the in-game browser doesn't work at all, this sounds like something entirely beyond my control. If the Steam overlay browser works for other games, then it sounds like this is a Stellaris issue, but if it doesn't work for any game, then probably a Steam issue.

Good luck!
And always worth mentioning: have you tried restarting your computer? ;)