PowerBI Report Server Not Accepting Upload

Hello, I am attempting to upload a report to PowerBI Server (May 2020), which was created in PowerBI Desktop (September 2020). I'm getting an error with the following stack trace: 2020-10-08 09:21:47.1662|ERROR|80|Failure in reportproperties| RequestID = s_da908ba8-e459-4960-ac9c-02ba1dc762c8 System.IO.FileFormatException: Input file or data stream does not conform to the expected file format specification. at Microsoft.PowerBI.Packaging.Storage.BasePowerBIPackagePart.ValidateExistsIfRequired() at Microsoft.PowerBI.Packaging.PowerBIPackager.Validate(Package storagePackage, Version& pbixFileVersion) at Microsoft.PowerBI.Packaging.PowerBIPackager.Open(Stream storageStream, Boolean& requiresConversionToReportLayout, Byte[]& securityBindings, Version& pbixFileVersion, Boolean skipValidation) at Microsoft.PowerBI.ReportServer.PbixLib.Parsing.PbixParserV1.ParsePbixFileIntoParts(Stream pbixFileAsStream, String requestId, String sessionId, Boolean areCustomVisualsEnabled) at Microsoft.PowerBI.ReportServer.PbixLib.Parsing.PbixParserV1.ParsePbixFileIntoParts(Stream pbixFileAsStream, String requestId, String sessionId) at Microsoft.PowerBI.ReportServer.WebApi.PbiApi.PbiApiController.d__38.MoveNext() --- End of stack trace from previous location where exception was thrown --- at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw() at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) at Microsoft.PowerBI.ReportServer.WebApi.PbiApi.PbiApiController.d__37.MoveNext() 2020-10-08 09:21:47.1662|INFO|80|Sending response. Response code NT AUTHORITY\NETWORK SERVICE 400, Elapsed time 0:00:00.0094633| RequestID = s_da908ba8-e459-4960-ac9c-02ba1dc762c8 Apologies for the poorly formatted stack trace - the post removed some characters. As far as I can tell there is no more recent version of PBI Server than May 2020. So I'm assuming this isn't a compatibility issue. Any thoughts?