Can any one tell me why an internal constructor on an associated class (many-to-one) causes this error: {"Creating a proxy instance failed"}
"Constructor on type ... not found"

when I make it public it works!?

Documentation says "All persistent classes must have a default constructor (which may be non-public) "

So why the error for internal constructor?

Hibernate tries to create a proxy subclass for your class when you use lazy-loading. So, a "protected internal" constructor should work.