Signup results in 404 page

hi there,

I am trying to create an account on the front end, via one of the buttons (all redirecting to the signin page). So I enter a username and email, press the Sign up! button and end up on a 404! page.

it doesn't seem to be able to find the /wp-login.php?action=register page.

I have my admin open in IE
tested in FF and Google Chrome.

when I set WP_DEBUG it generated the following message:
'Notice: Trying to get property of non-object in ../../../post-template.php on line 1275'

I am very much new to WP so I am still getting my head around the structure of things.

Another question I have (probably typical newby question):
how can I create / alter a form, e.g. I want to add extra fields for subscribers when adding listings?

Have been trying to find answers here, the WP docs, etc. for hours, but didn't find a clue. Please advise.

thanks in advance