2.0.x is far too old to expect anyone to assist in diagnosing. Even if you were on the latest 2.0.x release that's still 3.5+ years out of date.
I seem to recall a similar problem back then that was fixed, also a long time ago. I didn't see any specific references in any of the release notes, though.
From 2.3.x on it's even a completely different web server running captive portal connectivity (nginx now, was lighttpd back then)
The best path forward is to upgrade.