Hi Netshine,
I have ran into the same problem.
Joomla 1.7.3
nBill 2.3.0
When a client logs in nBill front-end and immediatly clicks logout the error "Invalid Token" appears.
When the client first clicks on My Profile or My Invoices and then clicks on Logout everything goes OK and the clients logs off.
When a client log on the URL is:
http://mywebsite.nl/index.php?option=com_nbill&view=nbill&Itemid=112 -> log off give error
When a client goes to My Invoices or My Profile and then clicks on Home the URL is:
http://mywebsite.nl/index.php?option=com_nbill&Itemid=112 -> Log off is OK
The difference in the URL is: &view=nbill
Maybe this gives you an idea where to look.