is this true ?
Can be. Depends on the hire company, but some of them certainly don't allow it.
problem is Germany - do they let hire cars across through to Poland?
As long as you have permission, sure.
i cant see any probs, as long as you're insured and the vehicle is legal, what problem could there possibly be?
They sometimes don't cover Poland, but it seems to be mainly German car hire companies that have an issue with it.