Can I keep my number after contract cancellation?

The white label indicates a payment problem, so check your bill/annual overview to see if there's an outstanding balance.

Without a German contact address, you can no longer have a contract with o2.

There is no outstanding balance, I have only recieved a new bill for this month though and its being deducted from my bank accout. I called customer support and they told me I can switch to cheaper tarif but line suddenly disconnected. 

But then there was one until recently. 🤷‍♂️

Hello ​@GiGiOver ,
thank you for contacting us with your question in the community. Welcome to the o2 community. You can terminate your contract at the end of the contract period and port your phone number to another contract (also with o2) and continue using it. The phone number can also be ported to a prepaid card. I'm sorry that you were unable to make the change online. Have you tried again in the meantime and did it work?
Kind regards, Bianca