IIS crashing seemingly at random during the day

Question

Over the last couple of weeks, my IIS Server has been experiencing some weird crashes. It's an IIS 10 running on a Windows 2019 Server.

I have 2 production web apps running on the server, both on .NET 9. One is an API and the other is an ASP.NET Core 9 MVC web app.

The web app uses custom error pages, so when the application level has a problem I have a branded error page throwing 500 errors.

Recently, users have been experiencing 500 errors that render with their browser standard pages. These can be spotted as a one off, but also has been spotted for 20 minutes. Restarting IIS solves these errors.

There have also been 2 occasions in 2 days, where the server just returns a blank page with no content. The only way to solve this has been to reboot the server.

There is something wrong, but I am not sure where to look. The Windows event viewer doesn't have anything obvious, and the IIS logs I've been able to find don't show me anything either (but maybe I'm looking in the wrong place).

I presume database connectivity to be OK, as an error of that sort would return my branded 500 page instead.

I use NewRelic and it looks like the server has plenty of Ram, CPU and storage. I do note that currently it is using around 190 threads per website. Not sure if that is a lot?

I could use any information on logs to check or potential issues I need to investigate that will help me solve the issue.

• Lex Li: When professionals talk about IIS crash, they are referring to IIS worker process (w3wp.exe) terminated unexpectedly, and the typical troubleshooting is documented in learn.microsoft.com/troubleshoot/developer/webapps/iis/…. What you described is more likely related to unhandled exceptions in those .NET 9 web apps, so you might reach out to the original developers of them and see if they can troubleshoot for you. You can contact Microsoft support via support.microsoft.com if you decide a support ticket is needed.
• Matthew Warr: I am the developer of the apps and it is not exceptions as they would be logged differently
• buymeacoffee: Hi Matthew, Collect memory dumps for a first-chance exception, when it occurs, then seek support from Microsoft, which may cost money.
• buymeacoffee: Or you can check the time period when the problem originally occurred, whether the server has been patched, or what software is installed, which can help us troubleshoot the problem.
• ErkinD39: In Event Viewer - Application folder, there should be an warning or error containing the crash details to verify that there has been an IIS crash. Only http 500 does not imply a crash. I think for error 500, it may help to get Failed Request Tracing of requests at problem times and see at which module the http request gets locked and throws http 500 error.