There are no notifications from Viber (version 6.6.1) on Windows 10 Mobile running on both Lumia 640 and Lumia 550

There are no notifications from Viber (version 6.6.1) on Windows 10 Mobile running on both Lumia 640 and Lumia 550, although on both devices the app is allowed in the background (even on power saver mode). The app quit sending notifications all of a sudden for no apparent reason. A fix to this issue would be appreciated.