ok, after some sleuthing, I find that .. if I turn off "notify by email when new users are registered" in Prefs/User info settings, then register.php does not croak. and users get past the second screen of registration process.

My site has multiple webmaster users, (2), and the first one get the email, but the second one doesnt, it's a malformed header that causes the 500 error.
So possibly a bug?? At least now that I have isolated the problem, and have a workaround (TURN OFF email notification of new users), I can spend a little time trying to sort this out.
But, if anyone of the devs out there want to try this:
Manually create 2 or more admins.
Turn on email notification of new users,
logout.
then register as a dummy user. Does page two, upon posting, croak and die - with an internal server error?

xoops 2.0.15 linux/apache2/php4

I have the same problem, apache2, linux, php4 though, and the server logs suggest there is some problem with the email, it says malformed header.
I am still looking into it, but if anyone has any suggestions... this would be the place
Neil.