When trying to authorise the plugin, you may see this error: 
Since authorisation also happens when the plugin opens, you may also see this message as you start a saved DAW session.
When you purchase a plugin, you can use it on up to 5 devices simultaneously. Every time the plugin starts, we check that you are allowed to use the product by:
- Checking the monthly payment status (or ownership of a perpetual licence)
- Checking the number of machines registered for your account and product.
If both of those checks pass we register the machine making the request. If you have exceeded the number of simultaneous devices, this error will show.
Machines are removed from the registry after 30 days of inactivity. So if you open a plugin with machine A every day and opened the same plugin on machine B once on 25/12/20, the number of registered devices will be 2 until 25/01/21 after which it will fall back to 1.