I provided a simple way to diagnose once-and-for-all if your firewall was blocking it.
Thank you for your attempt to help.
It turns out that it was
NOT a firewall that was blocking the requests. It was the
Web Protocol Filter in ESET Smart Security v9 Beta (it could be a problem with NOD32 and other versions). I actually had the ESET Firewall disabled completely.
You have to
exclude the J River processes from filtering and everything returns to normal.I still haven't figured out why ESET detected a threat in the requests/responses but it obviously doesn't like the method in which it is currently done.
This may be an important tip for any users of ESET products. If you want to add this to the troubleshooting wiki, I'm sure it will save others a lot of hassle and time. I was digging through logs, sniffing network traffic, analyzing packets, etc. before I came to this solution. I wouldn't want anyone else to have to go through any of this.
(Now I wonder what other issues the web filter could be causing with other processes.)