I installed version 16 to try it out. But, since ring queues is missing, I decided to go back to version 15.5. However, when I try to restore my backup I get the following error. "Product cannot be activated because license key is for a previous version. Upgrade license key or purchase maintenance from https://customer.3cx.com/orderform/default.aspx" It looks like the upgrade changed the license to version 16.0.0 and won't let me use it for 15.5. This is using the free standard license. How can I fix this?