We've seen reports of this exact error message, but there's no clear way for us as a browser maker to help debug.
Our best guess is that there's some web framework somewhere that's doing some non-standard verification that Hermit does not pass, presumably because of the tight privacy restrictions that Hermit enables by default.
If you have a way to contact the web admins, that would be really helpful. Even if you can figure out which specific web framework this is, we can try to look into it.