Extension Key is a unique identifier of M2E Pro instance. Each Extension Key undergoes validation based on Domain name and IP addresses. This is done to prevent inconsistent access to your data. The same Extension Key cannot be used for different domains, sub-domains or IPs.
Therefore, you will be asked to generate a new Extension Key if you have got a new IP address, renamed your domain, changed your Magento Server location, etc.
Please read how to get a new Extension Key and associate it with your current IP/Domain in this article.