The coins had 6 confirmations already. So i think its the server itself that wanted a fee. Because instantly after i changed the server no errormessage appeared anymore. Possible?
When this would be a security against some kind of ddos then it doesnt look like a far advanced one. I mean someone who wants to do this surely can change a line of code in an open source wallet to prevent the fee if its included in the client. Or if its the network itself then simply create a script that creates new addresses on the fly and sending back and forth the money to a new address everytime. Would take more processortime but would solve the feeproblem. But maybe this was taken in advance already.
hm well actually the bitcoin protocol enforces only 1 confirmation before a coin can be spent. But to spend your coins you need a miner to include it in a block. But it's considered standard within miners to include transactions with a sufficient fee for the size and coin age first.
Bitcoin clients usually force you to use a appropriate fee so your transactions won't end up unconfirmed. Normally electrum should let you ignore the fee, atleast for me it does so I don't really know why it didn't work with that particular server.
About transaction spammer attacks:
Ofc they can change the source code they could even use their own client but they still need a miner to include their transaction. Creating fresh addresses won't help anything. If i had 1BTC to spam the network and send it to myself then i have to wait for some confirmations before i can spend it for free again and how am I supposed to spam the network if I have to wait for a pretty long time before i can spend my bitcoin for free again.