Tankauth 404 on subdomain when submitting forms.

[eluser]dibs[/eluser]
I have just setup tankauth and am having 404 issues when I submit a register form.
The form returns a new register form on submission but in firebug I see I get a 404 for the url.
There is no validation information in the new register form.

I have the site in a subdomain and am removing index.php with mod-rewrite.

I can't figure out what is going on and would love if someone could help (i.e. point out something obvious and make me facepalm).

Cheers.