Oh, great! Thank you for showing interest and taking time to look into this. A second pair of eyes is always helpful and I definitely value your contribution.
Merely confirming the same observation, but in your environment, is a big consolation. I use Linux almost exclusively and have managed to reproduce the problem in a Windows 10 Home virtual machine too (using Python). But a VM and bare metal aren't the same thing. Not to mention your network configuration is probably different than mine (along with a slew of other things).
Just having another person see the same effect but on native Windows (with VB? C#?) is notable. And, knowing that I'm not alone is oddly reassuring. Burying oneself in the intricacies of a tricky problem can get awefully lonely sometimes, lol.
Al that said, today IB support acknowledged that "TWS appears to fail to properly clean up its own connection to the backend market data farm on behalf of the requesting API client, and this blocks subsequent API clients from making equivalent requests".
I'm hoping they'll get a fix in at some point. It's a low priority issue to them... but some priority is better than no priority :-) If I hear anything new I'll post it here.
Thanks again.
buddy