This error message appears when you have reached the maximum number of device authorizations for your Adobe ID.
With your Adobe ID, you can authorize up to 6 devices. Which for example can be your PC (Adobe Digital Editions), your eReader, your smartphone or tablet (Onleihe app).
If you see this error message, you have already used your Adobe ID 6 times to authorize a device. Please check if you really need all the authorized devices to read. For example, if you bought a new eReader, you can deauthorize your old reader in order to continue using your 6 authorizations.
It is also possible to reset all Adobe's IDs by contacting the Adobe support.