Again, thank you for your help. In the end, I had slightly wrong configs for my plugin, but now it seems to work. But since nutch makes no output on the commandline anymore, I can't find out if everything is correct in the end (readdb -stats).
I don't know why it is that way - I haven't changed anything. It would be create if someone would have an idea what to do now! My nutch version is 0.8. Best regards, Tobias Zahn