Hello there,
Please check Event Viewer on both RDS servers and clients, check to see if there is any relate event that may be helpful to narrow down the problem.
Besides, is there any change before this problem happens?
In general, we would recommended to check/install available Windows Update on both server and client, it may be helpful for resolving some known issues and improving the performance.
Available Updates for Remote Desktop Services in Windows Server 2012 R2:
Besides, configure appropriate timeout and re-connection may be helpful - Configure Timeout and Reconnection Settings for Remote Desktop Services Sessions:
https://technet.microsoft.com/en-us/library/cc754272(v=ws.11).aspx
Hope this resolves your Query !!
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