If this had happened in the UK, I am fairly sure that your 'contract' would be with the online TA, as they're the ones who took your money (regardless of whether they're acting as an 'agent' for KLM, or a 're-seller'). I don't know about German consumer laws, but I assume it's similar there. (This interpretation would also agree with what Mokshu has said).

I'm a bit surprised at the reference to force majeure, presumably meaning the bad weather. That could be a justification for refunding a ticket that was otherwise non-refundable, but it seems strange to use it as an excuse for not refunding a ticket.

Unless, of course, the online TA have something buried in their T&Cs somewhere, saying they won't give refunds for weather-related cancellations. It'd be worth looking for this - but I very much doubt this would be consistent with European consumer laws.

Did you pay by credit card? If so, and if you're sure the online TA owes you a refund, but they're not co-operating, then you could probably just dispute the charge with the credit card company. (Again, I'm familiar with UK law, but I assume Germany is similar - if you paid by credit card, then your 'contract' is technically with the credit card company, and as you didn't get the service you paid for (ie: a flight from A to B), they should refund you. I've only had to do something similar once, when an airline front office had happily agreed to cancel and refund a flight, but the transaction went AWOL in their back office accounts department. I raised this with the credit card company, and they refunded the original payment, no problems at all).

FWIW, I don't think EC 261/2004 is relevant here, as clause 14 ("meteorological conditions incompatible with the operation of the flight concerned") clearly applies. However other, more general, consumer protection legislation should presumably cover this situation.