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Reason: Wrong credentials. During the first connection, wrong credentials were provided and saved for Data Source.
Solution:
Click the File tab at the top left of Power BI Desktop.
Click Options and settings on the menu.
Select Data Source Settings.
Select the data source the “Access to the resource is forbidden” error arises for.
Select the Clear Permissions option on the drop-down menu.
Press the Close button.
Thereafter, you can re-enter the valid credentials when trying to connect the required data source through the Basic authentication.
Note: If you connect data sources from different instances, ensure to select the full connector URL during the authentication process:
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