Hi,
I just downloaded 6.10.3, installed it and attached the license without problems.
Please login to your account on acymailing.com and check if your domain is attached to your license. Unlik it from there and then try to add your license key again in your Acymailing.
If that doesn't resolve your issue, please raise a support ticket via Acyba's website: https://www.acymailing.com/contact/