Ok, I set up a second cold wallet in a different directory, moved the coins for the second masternode there and now the second masternode works fine.
So how can I manage several masternodes with ONE cold wallet? Is there something special I have to take into consideration? special parameters in the magnet.conf file?
there's a section in Cold Wallet Masternode Guide on how to run multiple masternodes from one wallet.
you should add new line for each masternode in masternode.conf
put following lines in magnet.conf
masternodesoftlock=1
staking=0
do not move your collateral coins
start masternodes one by one by running the following comand in wallet console:
masternode start-alias YOURMASTERNOD1 YOURWALLETPASSWORD
masternode start-alias YOURMASTERNOD2 YOURWALLETPASSWORD