We are allowing two activations for one license, then if you activated only one computer, you have a remaining activation. Then just download the full installer at our downloadcenter and do a clean install on your other computer.
Try again later, the server may be overloaded by activations requests.
You should disable kapersky during the upgrade. Check in your ZBrush 4R4 folder if you have an upgrader.exe and check the date. If it’s from today or yesterday, execute it again. If not (Kapersky may have deleted it), download in our download center the full 4R5 installer.