This error message may occur when using iStudiez Pro on Android due to the issue with the Certificate we use which recently occurred. Although the Certificate works OK on iOS, macOS, and Windows, it stopped working on Android.
We are doing our best to resolve the issue.

This issue may occur on iOS, macOS, and Windows, if you use a public Internet connection (e.g. in a library or college), the network there may have some firewalls or other blocks installed. 

Those blocks may prevent iStudiez Pro from connecting to the Cloud Sync server. 

You may need to contact your local network administrator for assistance or try to use the Cellular Data instead of Wi-Fi connection when on mobile to be able to sync.