Hi spouseele, and thanks for the reply!
My clients are configured as both SecureNAT (they get the ISA server's internal IP as default gateway through DHCP), and I also have the firewall client installed (just for testing purposes).
The HTTP Redirector is configured to "Redirect to local Web Proxy service".
I did look at the logs, but as you rightly said, I have some problems making sense of them. I'll look at the links you gave, thank you very much.
Maybe it will help if I'm more specific:
1. On one client, with the firewall enabled and logged in as a normal domain user, I cannot complete "authentication" on some web sites that are SSL secured. But on others I can. When I disable the firewall client, I'm successful. On some other web sites, e.g. www.clanbase.com, I get an "unformatted" web page, i.e. it seems as if things like style sheets are not applied/downloaded. I also get an error saying that a script on the web page has caused an error, and the error box tells me that an "object [was] expected". This web site uses PHP, but it's no more than your average scripting error. The point is that the scripting error occurs in the first place.
2. When I log onto the same client as domain administrator, with the firewall client enabled or disabled, I don't have the above-mentioned problems. I get the properly formatted web page, there's no scripting errors, everything seems absolutely fine.
3. On my second client, I do not have any of these problems. I get access as any user, with or without the firewall client.
So the problem seems to be specific to just the one client machine, right? So last night I reinstallled the whole client computer (it was due a spring clean anyway), but the problem remained. I restored my server to an image I made about 3 weeks ago when everything was still fine, and it still doesn't work.
I'm beginning to think that this one cannot be solved easily, and maybe I should just live with it. And while these types of requests get handled through the Web proxy service, why on earth would the firewall client configuration make any difference?
This is really strange, since I haven't made any configuration changes except for installing Norton Antivirus 2003 on the client. It literally seems to have just stopped working without any outside help. I update my server and clients frequently with the latest Microsoft patches, so I don't know if an update I applied somewhere might have caused this problem. Even so, it is still just the one machine. That is what doesn't make sense to me.