Howard Miller

Could even be an old-fashioned permissions problem. From experience, this can be a pain as you don't get much indication of the underlying cause in the errors. A careful "is everything done right" check over is the place to start.

Do you have any optional plugins (that may not be compatible with PHP 8.3)?

Ok - that's just the access log. What we *really* need is the log of whatever caused that 500. It should be in the error log. Unfortunately, I know nothing about Windows/IIS so can't help with the specifics. I have a very vague recollection that IIS doesn't log errors by default - but I could easily be talking complete nonsense. Without knowing the actual error message, we're really struggling here.

Unless I'm missing something... there's no Error 500 in the log line you have shown. It's a successful HTTP GET request...

That sounds like files were transferred by FTP set in the wrong mode. That's all that comes to mind.