New IIS install suddenly giving 500 error on basic static web site

I maintain a basic static website for personal use, and since my ISP stopped hosting it several years ago, I've run it off of my home computer. I just put in a new SSD this week, and installed Windows 10 fresh. Activated IIS, set up the basic "hello world" site, and then copied over my site's files. Everything seemed to be working fine.

Today it's not working fine. I'm getting a 500 error:

Calling LoadLibraryEx on ISAPI filter C:\Windows\Microsoft.NET\Framework\v4.0.30319\\aspnet_filter.dll failed

Detailed Error Information:

• Module: IIS Web Core
• Requested URL: http://localhost:80/
• Notification: Unknown
• Physical Path: C:\inetpub\wwwroot
• Handler: ExtensionlessUrlHandler-Integrated-4.0
• Logon Method: Not yet determined
• Error Code: 0x800700c1
• Logon User: Not yet determined

I checked https://docs.microsoft.com/en-US/troubleshoot/iis/http-error-500-when-you-visit-web-site for the error code but it wasn't helpful; it says "Make sure that the script-mapping points to the ISAPI .dll file that can process the request" but the handler in question doesn't point to any DLL file -- it points to System.Web.Handlers.TransferRequestHandler.

(I'm sure I'm missing something here, as while I'm a software engineer I'm not a trained webmaster and have only a rudimentary understanding of the terminology involved. Go easy on me!)

Any idea what's going on? Or what could have changed to produce this error without me touching the IIS configuration?