Hi Joerg,
You can only be connected to two devices at the same time with Boxcryptor Free. If you did not log out from a device, you can manually unlink a device in our web interface here:
https://www.boxcryptor.com/app/device/overview
Best regards,
Robert
PS: You get one additional device for free if you refer five people to Boxcryptor using your referral link which is shown in "My account" in our web interface.