This question has been answered by tm2000. See the first response.
I think someone should stop by soon with some real knowledge of this problem, but for now I did some research on this.
Let's see your confirmation snippet pls, after copying it over here pls put in the &errorPage=`X` variable to try to get more info.
Also, is the link confirming the registration? Just because it is going to 404 doesn't mean the user isn't being activated, pls check that as well.
The resource at the URL they get sent to should contain just the ConfirmRegister snippet tag. It's easy to get confused about the Register, thanks for registering, ConfirmRegister, and "you are confirmed" pages.
Ah great you got it sorted. Yeah caching issues can be a pain, its so important to flush everything out.
By the way the &errorPage variable allows you to specify a custom error page, this is to see if the snippet is firing, and I think it can provide more info too