Error occurred - status 503 returned

Question

I have been encountering:

Looking in the Chrome console when this happens, I see:

That particular example occurred on Database Administrators when voting to delete a comment. I have also had it when voting on questions and answers.

I have had these red box errors in the past on the odd occasion, but it seems to be much more frequent over the past day or two. I've only recently learned about the Chrome console (thanks Shog9 & ArtOfCode), so I can't say if past errors would have had the same detail.

While editing this question to respond to a comment, I received:

with:

So it seems not to be limited to the site where I am a moderator. Logging out of Stack Exchange everywhere and clearing all my sessions and cookies did not resolve the problem.

Latest example:

Answer 1

I originally thought it was an internal network blip between our load balancers and the web servers. It turns out that another explanation is likely correct.

First of all, we've upgraded SQL Server to 2016 last week. This is great for a number of reasons, but we (and by "we" I mostly mean Nick Craver) have been chasing performance issues since then.

Secondly, having switched to fastly, slow performance might be reported to users as 503, whereas we don't see corresponding exceptions in the logs.

We're on it.