Error 405: There are too many devices connected to my account.

In addition to the option to disconnect a device one at a time, you can also easily remove ALL devices in one go. This option can be found via:

iOS: Settings / Premium Status / Assigned to account / Change password

iOS:(for non-active Premium accounts or non-existent account connections) Settings / Premium Status / Activate Premium Service / Restore Premium / Change password

Mac:

Preferences / Account / Manage Account

Here please enter your credentials (if a popup appears stating you’ve already registered please remove your device from the account and re-register it again afterwards.) Due to safety reasons your password will be requested, otherwise the remove button will not respond.