Irrespective as to whether the "rules" cover such an eventuality as that occurred, most company's "terms of business" will expressly include a right to recover goods, services or money that the customer was not entitled to - such as additional premium credits that would normally have cost "real money" to purchase. So I think Innogames were completely within their rights to recover premium gold that was inadvertently made available by people exploiting an oversight by the developers (I too took advantage of it by joining 10 worlds).
However.........
1. The only reason that I had sufficient credit for Innogames to actually be able to recover this act of "generosity" was by nature of me having purchased premium in the first place. This places me at a disadvantage to players who just grabbed the gold and who have already spent it, which is unfair.
2. On the day it happened, I specifically recall that the oversight was identified by a player very quickly and referred to in a posting in the Forum - and a senior moderator replied to it by a "wink" and referring to Innogame's "Christmas Spirit". This posting has now disappeared. By nature of the acceptance of the apparent oversight by a duly authorised "agent of the Company" (whether paid or not is of no relevance), then I believe that Innogames had duly accepted the position of having given out a "freebie", and now to have rescinded that acceptance.
3. Whilst there is no requirement for there to be an appeals system, and there have been numerous references above to "sending in a support ticket" if anyone thinks that what has been removed from their premium account is not in accord with how much was added to a players account by using multiple start ups in different worlds, it really was beholden on Innogames, if only for PR purposes, to have dealt with the problem quickly - within a day or two of the event - and not wait for some 3 weeks before doing anything about it, AND to have sent round an announcement as to what and why they were clawing back premium, and how they were calculating it, and when it was going to occur. They are quick enough to send round messages encouraging players to join new worlds/different game products, so they are quite capable of having done this. An "apology" also would not have gone amiss, even if the Company felt that they were more sinned against than sinned - a few placatory words never cost anyone anything.
To just reduce the amount of premium in my account without any explanation or advise just does not strike me as being very professional or indeed "acceptable", irrespective as to where the Company is based (& therefore subject to its own countries laws - not being German, I have no idea what EU/German Law might say about this subject). We live in an international world, but basic good manners transcend borders, and I would ask the MD to look to how he would feel if the same thing had happened to him - I feel sure he would not be very impressed with how his Company has dealt with the situation.
Innogames depends on the goodwill of the people playing their games, and the livelihood of their 200 or so employees depends on us, players from all over the world, paying "premium" to play their game(s). This sort of thing will, unfortunately, not do their reputation any good.