See: Uploads Stopping for Projects with Large Files #4572
https://github.com/BOINC/boinc/issues/4572

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I still think it would be helpful to identify exactly which of the many possible 'transient upload errors' is being encountered in your use case. You referred to my #3778, but that was concerned with the problems which occur with the boinc backoffs when files fail to upload: The cause of that at GPUGrid is well known - a DDoS block on the connection when multiple computers try to connect to the server from the same IP address, in too short a time interval.

Your issue #4572 gives an example of a simple 'transient HTTP error', but the extended http_debug log shows an error-free 'Finished upload of ...' log, which takes us no further forward.

I also run WCG, and I've seen examples of an error "no server is available to handle your request" - which I don't think I've seen at any other BOINC project.

You also mention that you think that the errors are connected with the uploading of 'large' files - can you quantify that? I've recently handled problems caused by attempting to upload files larger than 150 megabytes at CPDN, and 500 megabytes at GPUGrid. Are your files in that sort of size range? Also, in my experience, once a connection has been established and data has started to flow, in general uploads complete: the exception being projects which transfer uploaded files to backing storage, which sometimes accept the whole file and then generate an http gateway error because the backing store is full or offline. Does your error occur at the start of the transfer (in which case the size probably isn't implicated), or does it occur part-way through, or at the end?