I just discovered that the random but infrequent stuck file transfers I've been seeing over the past month or so are being caused by AVG anti-virus's identity theft protection.
It's likely some kind of bug in AVG. The executables are certainly not infected.
If you use AVG, I *strongly* recommend configuring AVG to ignore the C:\ProgramData\BOINC folder, and also disable identify protection. You can't tell the identity protection system to ignore a folder, so you have to completely disable it.
Or switch to a different anti-virus product.
My lucky number is 75898524288+1