It seems that when an account has Read Only access to tickets, it is not allowed for him to list tickets via the API.

Any person who can read tickets in a project should be able to access the tickets in that project via the Unfuddle API. If you believe this to be an issue for a user in your account, please email us at support and we will be happy to investigate any issues for you.

After some further investigation, it turned out that the permissions were not set correctly. Our fault, sorry.