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 / Account / Manage Account

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.