SQL Server timeout expired.
Try this action again. If the problem continues, check the Microsoft Dynamics 365 Community for solutions or contact your organization's Microsoft Dynamics 365 Administrator. Finally, you can contact Microsoft Support.
Why does this error occur?
This type of error can be confusing, especially in the majority of cases where app builders build model-driven apps based on Dataverse, and SQL Server doesn't enter the equation.
The cause of this error is that Dataverse is based on SQL Server, and uses SQL Server as its underlying data source. When there is slowness or a problem with the underlying connector infrastructure, this will appear to users as a SQL error.
Therefore, we can think of a 'SQL Server timeout expired' message as a generic message that appears when a problem occurs with the connector/Dataverse layer of the Power Platform.
How to fix the "SQL Server timeout expired" error
Unfortunately, there's not much we can do to resolve this problem, as it is mostly outside of our control. In many cases, the problem will go away by itself after some time, particularly if the fault is related to excessive load on the connector infrastructure.
For a quick resolution, the best thing to do is to raise a support ticket with Microsoft. In my specific case, they used my session data to detect a regional outage that impacted connectors, and they were able to increase the capacity in order to fix the problem.
Here's their summation of the issue.
After investigating additional telemetry, we identified a database infrastructure component reached a capacity constraint limitation, causing API calls for connectors to fail. To mitigate the issue, we increased capacity for the impacted database infrastructure component and confirmed via telemetry that Power Apps which utilize connectors are no longer failing