My ASP.NET Core website includes a bootstrap carousel with 35 small PNG image files (each around 4 kbytes). The first 20 of these images load but the next 15 fail to load with 503 error codes. These numbers vary but it's always successfully displayed followed by unsuccessfully displayed images. They're not interleaved. If I browse to these failing image files directly they display correctly. Looking at the network traffic, I see for a successful image the server response header is "Microsoft-IIS/10.0". For a 503 response the server response header is "Microsoft-HTTPAPI/2.0". I'm not sure if this is relevant though. If I run the same application locally on IIS there are no errors. What could be causing these 503 errors? How do you debug this type of issue?
503 errors can be the result of your application exceeding application pool limits such as CPU usage or memory. It can also be caused by exceeding the HTTP concurrent connection limit for your site. You can open up a support ticket so that we can review the server logs to see if it's one of these issues. Please note that not all 503 errors are caused by the above, but they are the most common reasons.
