Did you recently complete an offer worth 500 tokens?

Sometimes if you haven't followed the rules for obtaining them they deduct the tokens back from your balance after a short period, so if you were awarded 500 tokens and then spent them, you'll have a -500 balance.

The reasons I can think of are use of proxies or cancelling a paid offer before the required period of subscription time.

But as Khris said, try support@nordeus.eu

Although I suspect the problem lies with the token provider.