Log in to the machine not able to communicate, open Windows Services and restart the following services:
-Shadowprotect Service 
-Storagecraft Image Ready 
-Storagecraft Raw Agent 
-Storagecraft Shadow Copy Provider 
After that, go to the Datto portal, click on the Protect tab, find the server on the list and click on Configure Agent Settings. Scroll down to the bottom and click on Repair Agent Communications. If It is successful, go back to the Protect tab and Start a Backup.