I clicked activation in the email but couldnt activate. The Settings page tells me: unknown user - c(...)

Allow your iSafari (the webbrowser of your iPhone or iPod is called iSafari) to accept cookies. Solution: Go to Settings on your iphone or ipod touch main Menu, click the Safari icon, choose: "Accept cookies" - "From visited", or "Always". If you choose "Never" your clips are limited to a length of 20 seconds.