Upload Error reading MIME multipart body part

Strange behavior on new install of PBIRS (May 2022): using the API call to /model.upload (efficient binary upload) I can uplaod a simple direct query report but if I convert it to import mode the upload fails after about two minutes. Portal log:

2022-10-16 19:44:42.6877|DEBUG|10|Creating LocalFile as stream destination: C:\Windows\ServiceProfiles\PowerBIReportServer\AppData\Local\Temp\PbixUpload_7eebf8ac-d518-4eb8-8fa2-6b74f226f513.large| RequestID = s_5fde1a09-ad17-4e46-80ea-3366586ea918 
2022-10-16 19:46:39.2839|TRACE|22|The file upload has been cancelled.| RequestID = s_5fde1a09-ad17-4e46-80ea-3366586ea918 
2022-10-16 19:46:39.2839|TRACE|22|OData exception occurred: Microsoft.ReportingServices.Portal.Interfaces.Exceptions.UploadFileCanceledException: The I/O operation has been aborted because of either a thread exit or an application request ---> System.IO.IOException: Error reading MIME multipart body part. ---> System.IO.IOException ---> System.Net.HttpListenerException: The I/O operation has been aborted because of either a thread exit or an application request

Most of my reports fail to upload the same way. But some (very few) have no issue at all. Never had this issue on the old server. Does anyone know what the call to model.upload would fail with the above error?