Hi Siebe,
There’s no need to worry, this is likely an issue with the wallet interface rather than the network. Firstly check that you have the latest wallet version (v1.3.2), you can update if required at Wallet and then try to unstake (ensuring you have 0.6XRD in your unstaked balance).
I would also suggest that if you still get this error, try to unstake slightly less than the maximum