It happened to me too... sometimes when i add something it freeze(opensuse 11.4). I deleted all sites, clients, dns zones, and users i have created before.
I ran the following command to clear mysql databases:
mysqlcheck --all-databases --auto-repair -p_sqlpassword_
then i started by adding 1 client(directly under admin, no resellers) following by 1 site for that client, following 1 domain for the mail(property of the client), and then started adding mailboxes.
It didn't freeze anymore, but anyways, when it freezes(being only the control panel) i restart apache2 and it comes back. fortunately i only have 1 domain to host, nothing complicated. I hope it works for you too.