Hi Pedro,
I asked the technical persons at the customer (they solved the problem) for more details on this topic. They could not remember a lot of what was wrong and what they did. They could only find a link towards a technet page.
technet.microsoft.com/.../aa497043.aspx
There are some remarks about older AD versions and two-way trusts.
I hope this topic will provide the information you need to solve your problem.