The problem is not just limited to Windows. I now have one stuck on my Ubuntu machine.
Stuck Ubuntu upload

That one just uploaded successfully and is marked "valid".

All's well that ends well.

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My machine's backlog cleared out today.
A big THANK YOU to all the hard working folks that resolved this.

P.S. Some of us are dying to know what caused such a strange problem.

Looks like it's been fixed, but I'm still curious what the problem was. My stuck-on-uploading results started clearing yesterday, and the first four machines I've checked are all good to go now.

Mine started uploading from exactly the same time, so I'm speculating that whatever got fixed happened on the server side.

I'm also making the assumption that whatever change was made on the server side to enable the uploads was documented because, while rare, it has happened before.

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A note from our systems engineer:

UW IT got back to us and said that they removed the filtering on their equipment. And indeed, since around 11 am all five BOINC web servers have been receiving results OK with the minimal failure rate, about 10 in the last 6 hours vs. 15000/day.

The issue seems to be resolved for now but let's keep an eye on things. UW IT provided no information what triggered this block, so it might happen again.

Luki

Our systems engineers have been working hard trying to figure out what was causing this odd behavior. It looks like it turned out to be out of their control since everything on our end (servers, local network, etc) seemed ok from their diagnostics and analysis but they contacted the right department, University of Washington IT, to figure out the issue and get it resolved for now.

Kudos to Luki, Darwin, Keith, and Patrick!