I also face the same problem! Did you find the solution? Can you share with us?

Thanks.

I solve this problem overriding signup_url method. 

Requisite: you have to assign a specific website to the partner (linked to portal user).

Original:

def _get_signup_url_for_action(self, url=None, action=None, view_type=None, menu_id=None, res_id=None, model=None):

...

signup_url = "/web/%s?%s" % (route, werkzeug.urls.url_encode(query))
if not self.env.context.get('relative_url'):
signup_url = werkzeug.urls.url_join(base_url, signup_url)
res[partner.id] = signup_url

return res

My code:

signup_url = "/web/%s?%s" % (route, werkzeug.urls.url_encode(query))
if not self.env.context.get('relative_url'):
# this three lines make the magic, we ask if the contact has a specific website assigned,

# so we use partner_id.website_id.domain instead base_url from database parameters.

if partner.website_id:
signup_url = werkzeug.urls.url_join(partner.website_id.domain, signup_url)
else:

signup_url = werkzeug.urls.url_join(base_url, signup_url)
res[partner.id] = signup_url

return res

same here..

Same problem here!