The expiration period for the Client Secret cannot be more than two years from now.

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Instructions

For security purposes, Microsoft removed the ‘Never Expire’ option for the Client Secret Expiry in April 2021.
The expiration period cannot be more than two years from now.

An alert will be displayed on the Citrix DaaS - [Full Configuration] - [Hosting] console before the secret key expires. But, if the secret key expires, there will be connection issues.
It is necessary to get the application secret in Azure and change it in the Add Connection and Resources wizard.

※ Citrix recommend that change the keys regularly for security purposes. Be sure to save the key because the key cannot be retrieved later.

NOTE：
Copy the client secret value (NOT Secret ID) and the expiration date, and edit the corresponding connection and replace the content in the Application secret and Secret expiration date field with the values you copied.  

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The expiration period for the Client Secret cannot be more than two years from now.

More details on the feature from Citrix and Microsoft Documentation:

Get the application secret in Azure
https://docs.citrix.com/en-us/citrix-daas/install-configure/connections/connection-azure-resource-manager.html#get-the-application-secret-in-azure

Manage the application secret and secret expiration date
https://docs.citrix.com/en-us/citrix-daas/install-configure/connections/connection-azure-resource-manager.html#manage-the-application-secret-and-secret-expiration-date

Client Secret expiration now limited to a maximum of two years
https://devblogs.microsoft.com/microsoft365dev/client-secret-expiration-now-limited-to-a-maximum-of-two-years/