I must tell you that there could be many reasons for this behavior, but in this blog, I would talk about a cause which was due to SQL Server. Let us learn about how to fix error related to the Notification Service.
For one of my client, he found that users logging in or logging out were getting “Please wait for the system Event Notification service” and the server must be rebooted to resolve this issue. Below is the screenshot.
Check if you are running SQL Server Management Studio on the machine or any other software which uses Visual Studio Shell? There are some issues with System Event Notification Service (SENS) related to VS Shell and my client has opened a ticket to investigate with Microsoft. A workaround for this was to kill the SSMS process in task manager, then SENS starts working again.
Here is another workaround to create registry key. Basically, it is to disable the Telemetry for VS shell which is used by SQL Server Management Studio
If you find some other solution, please write in a comment and share with others. Let me know if you have seen this error before in your production server?
Reference: Pinal Dave (https://blog.sqlauthority.com)