It could be the special characters in it. Try adding "SecureURL = no" to the virtual host configuration of the openwebmail site.
I just tried an online demo of openwebmail. I got a lot of errors. If you have any trouble with openwebmail, you could give
Hastymail2 [www.hastymail.org] or
Roundcube [roundcube.net] a try. I have a very good experience with both of them.