I've installed codoPM in my joomla site, but it does not seem to work for a registered user, only for a super user.
A super user has sent a message to a registered user, and the registered user cannot see it - there is nothing in his inbox.
And when the registered user clicks on the compose tab, no fields unfold so as to write a message.
Any thoughts why this might be happening?

Did you make any changes in the settings for codopm in the Joomla backend ?

No I did not make any changes in the settings for codopm in the joomla backend. There is not much to change...

Also even if I give super users rights to this registered user, he still does not have access to any messages, neither do any fields unfold so as to write a message when clicking the "compose" tab.

Can you give us your website address with 2 test accounts ?