As it says within the error message, this is an error from PracticeAdmin Scheduling Applet (the desktop app, not the Web App). The message occurs when PracticeAdmin Scheduling Applet loses its connection to our servers. 

That can happen in two primary scenarios: 

1. PracticeAdmin Scheduling Applet hasn't been used in a while and its session has timed out.

2. The connection between PracticeAdmin Scheduling Applet and our server has been interrupted because of an issue at your data center,  at our data center, or with the VPN connection between the two data centers.