This is most likely a mapping file mismatch.
Check that your inversed-by and mapped-by attributes are set correctly.
Also check that your one-to-one, many-to-one, one-to-many,
and many-to-many are correct.
Most common seen with empty mapped-/inversed-by,
two sides both trying to be the same association
(many-to-one, on both for instance), or by mismatched attributes.
one-to-one bi-directional should have inversed-by and mapped-by
one-to-many should have mapped-by
many-to-one bi-directional should have inversed-by
many-to-many bi-directional should have inversed-by inside the join, mapped-by outside the join