M2E Pro error "The listing was locked by another process"

M2E Pro eroor “The listing was locked by another process. Please try again later” means that there’s some listing activity performing at the moment. So this listing is locked and it is not possible to perform any action on it until the current activity is finished.

 As a solution, you should just wait and try again later.

Usually, this situation happens when automatic synchronization is performed on the listing and you're trying to perform some action at that moment as well.