This message can appear because:
1) You are trying to use the same Extension Key for the different domain, sub-domain.
Each Extension Key undergoes validation based on Domain name and IP addresses. This is done to prevent inconsistent access to your data. It also protects from the risk of the synchronization being performed from several instances simultaneously.
2) You have reinstalled M2E Pro in the place different from the previous one. For example, you were using this Extension Key on /var/www/magento/, then on the /home/user/public_html/.
Please contact M2E Pro Support Team to solve the problem.