When you enable multithread currently it only affects the download test. The upload test doesn't reference any of the variables related to enabling multithread.
Your issue is odd. Do you have any new Internet security or other programs that may affect the output of websites in your browser? Have you tried a different browser? The odd part is that you're able to trigger this by disabling multithread. It really shouldn't be possible.
Clear your browser cache and cookies and try a different browser. That may provide more clues.