The error is in SSRS, you need to check SSRS logs and event viewer where SSRS can post errors as well.
Here is how to find SSRS log Reporting Services Log Files and Sources - SQL Server Reporting Services (SSRS) | Microsoft Learn
Check the errors around the time you see it.
Last time I saw this error SSRS was printing in event viewer "“The report server has detected a possible denial of service attack. The report server is dropping requests for service from the IP address…” but that was intermittent error and your case may differ.
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