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But the fact that I bought the EOS with ether from my MetaMask, and claiming them after with my MEW. Is that even possible? Or has the EOS been send to my MetaMask, even though I had to reject it because of lack of fund?
At the moment, there are no EOS in my wallet.
How do I register the eos public key?

it is possible just take out from your metamask the private key and put it into mew , it's all virtual look at @Sandwich blog..

Aha. Thanks. Actually, yesterday when I went back to check the distribution app on EOS, the unclaimed tokens were back. So I paid the gas, and were able to transfer them afterwards to my MEW account. Must have been an error happening because of all the usage on their system or something I guess. But thanks for all your help :)