iMac DV+ (Summer 2000) "Security Failure. The server reply is invalid."

I think it's because that's a very old browser.

Try loading this page: https://www.tic.com/index.html

That's an antique webpage, so it'll be interesting to see if it loads. You'll need to find a slighter more modern browser, which is a challenge in OS 9. iCab has some older versions posted that may still be useable. See http://icab.de/download.html

Yeah, true. Personally I wouldn't be be eager to browse on such an old system. If I really wanted to use such an old Mac for anything but running old software, I'd look for a still supported BSD or Linux to install in place of Mac OS.

Today, many web sites are secure. My tests using an early iMac G3 and Mac OS 8.6 indicate that all applicable web browsers are limited in one way or another. Internet Explorer 4.01 (PowerPC), iCab 2.9.9 (PPC) and iCab 3.0.5 (PPC) seem to have general difficulties connecting to secure pages, but the browsers may work with plain http sites (try, for example, knubbelmac.de for a comparison). Classilla 9.3.3 connects to many secure sites, but problems can appear when sign/log in is required (JavaScript will need to be separately enabled; see also Preferences). Many certificates are now obsolete/expired. Even if it is possible to connect, do not expect web pages to look well or load as intended.

The situation should not be too different with your iMac DV+ and Mac OS 9.2.2.