Getting random/intermittent 503 errors on my local IIS website

Turns out that somehow my 'Connection Limit' setting had been set to 1.

Fixed this by opening IIS Manager, selecting the website, clicking 'Advanced Settings' (in the sidebar), expanding 'Connection Limits' and changing 'Maximum Concurrent Connections' to 10.

To get a better understanding of why some requests fail with HTTP 503, you could set up Failed Request Tracing.

Add a rule matching status code: 503, and add all available providers.

If you don't use the Default Website for anything, just go ahead and remove it.