I ran into this same activation issue before, so I know how frustrating it feels. That “internal server error” almost always means the token on CyberSource’s side expired or their signup server was having one of its random hiccups. What finally worked for me was requesting a fresh activation email and opening the new link in an incognito window. The first link never worked, but the second one went through right away. If that still fails, just re-register using the same email and it usually sends a clean token that activates without trouble. Their sandbox setup breaks more often than it should, but a new link has always fixed it for me.
