After Magento Team made the announcement about Magento Connect closure, the website was eventually closed down on the 15th September 2017.
It means that Magento Connect can no longer be used to install/upgrade the Module automatically. Instead, M2E Pro Team highly recommends that you use Magento Marketplace as the most preferred option.
Check out other options of the Extension installation/upgrade. Use the alternative ways only in case of extreme necessity when the automatic installation/upgrade via Magento Marketplace cannot be executed for some reasons.
The detailed instructions on how to install/upgrade M2E Pro Extension for Magento v2.x.x can be found here.