The permissions granted to user '' are insufficient

I installed Power BI Report Server – SQL Server 2025 Standard, product version 15.0.1121.109, on the primary node only of a Windows Failover Cluster SQL Server 2025 (no AG) – OS Windows Server 2025 Datacenter.
I configured the SMTP settings in the Email Settings section of the Report Server Wizard.
I created a shared PostgreSQL data source that connects using a username and password, and then created a paginated report using that data source.
The permissions for my AD user on the Report Server folder containing both the data source and report are all: Browser, Content Manager, My Reports, Publisher, Report Builder.
I created an Email distribution subscription for my report, but when I run the subscription either manually or on schedule I get the below error:
 
Failure sending mail: The permissions granted to user '' are insufficient for performing this operation.Mail will not be resent.