Classifieds and SuperCache sign up fails

Hi,

I have set up a site with Classifieds plugin.

I also installed SuperCache plugin, (I wanted to use caching as its on a shared hosts with very limited db connections so wanted to limit the calls to the db)

I used SuperCache as I have never seems to get W3TC working on multisite.

The site SEEMS to work well except signup, where signup-.php give the error 'please try again'

If the sign up is from http://sixry.com/classifieds/signin/?redirect_to=http%3A%2F%2Fsixry.com

But if the sign up is directly from http://sixry.com/signup.php
then it is OK

So I am guessing that the cached page http://sixry.com/classifieds/signin/ is sending a cached nonce or something.

Any ideas how to solve?