Can anybody tell me why I don't have dataconnection even if the 3G-logo is showing up?

Sometimes the arrows 'up' and 'down' above the 3G are lightning up for a second, but are most of the time greyed out..and Nitdroid says I'm not connected. So I can't visit internet, can't use whatsapp etc.

On Maemo, It directly connects to 3G and I can browse internet, sync mails etc. etc.

Anybody an idea what I did wrong?