Extension Key is a unique identifier of M2E Pro. Each Extension Key undergoes validation based on Domain name and IP addresses. This is done to prevent attacks and inconsistent access to your data. The same Extension Key cannot be used for different domains, sub-domains or IPs.
If you’ve moved your Magento to a new web server and/or new host with a different IP and/or domain, you need to obtain a new Extension Key for your Module. The detailed instructions can be found here.