An 8.0.2 UML model successfully generated a schema in 8.0.2, but it generates errors in 8.1.
At 8.1, the geodatabase traps for more modeling errors than at 8.0.2. The following are common modeling errors that were allowed in 8.0.2, but that will now generate errors in 8.1:
Recursive relationship classes. The same class cannot be both the origin and destination class in the same relationship class. Relationship classes must be between 2 different classes.
Multiple relationships between the same classes of which one is composite. If two classes participate in a relationship class with one another that is a composite relationship class, then no other relationship classes can exist between those classes. However, two classes can participate in multiple simple relationship classes with each other.
A single class acting as the destination class in multiple composite relationships. A class can be the destination class in only one composite relationship. A class can be the origin class in multiple composite relationship classes, or it can be both the origin or destination class in multiple simple relationship classes.
Fix your model to remove these modeling errors and re-run your model.