If you use Windows system notifications for eM Client and eM Client is already running, you can encounter this error when trying to perform an action directly from the pop-up:

Communication with the previous instance failed. The other instance has to be ended in order to start the application.

This happens if you run eM Client in administrator mode, which is not compatible with Windows notifications. To avoid this issue, make sure to run eM Client as a regular user instead of 'as admin'.