Product:
Microsoft PowerBI report server  Version 1.10.7737.32652 (January 2021)
Microsoft Windows 2016 server

Issue:
Some of the powerbi reports does not refresh from the portal or on schedule. You get a error message like this;

2022-01-19 : |ERROR|63|Error Processing Data Model Refresh: SessionId: , Status: Error Failed to refresh the model, Exception Microsoft.AnalysisServices.OperationException: Failed to save modifications to the server. Error returned: ‘COM error: Microsoft.PowerBI.ReportServer.ASEngineManagedRoot, Datasource … is not found..
COM error: Microsoft.PowerBI.ReportServer.ASEngineManagedRoot, Datasource … is not found..
The current operation was cancelled because another operation in the transaction failed.

The powerbi report can be refreshed from PowerBI for Desktop. Also the report works from a different PowerBI server.

You can check error messages in folder c:\Program Files\Microsoft Power BI Report Server\PBIRS\LogFiles

 

Suggested solution:

Restart the windows PowerBIReportServer service, to release memory or cache that is used by PowerBI server.

 

More information:

ERROR|19|Failed to get connection string for model | RequestID =  ClientSessionID =  Microsoft.PowerBI.ReportServer.AsServer.AsConnectionException: Failed to get connection string for model

error in the log file can be because you have 2 data sources in the model pointing to the same database, where the database name was all lower case in one source and upper case in the other. Use the advanced editor in PowerBI desktop to rename them to be unique and then there was one source and data refreshes fine. You must use the same letter format on all fields.

https://docs.microsoft.com/en-us/power-bi/report-server/scheduled-refresh