I've never heard of anything like this.
Me either, so I tried it a number of times to make sure that MC was indeed a catalyst here. I'm betting it's not the root cause, but it's involved somehow. I am using the default MC port ranges, nothing on 80.
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How have you determined that there is no WAN connection?
When the laptop seemingly botches the WAN, is this true for other systems as well, or just the laptop?
It goes down for the whole house, seemingly at the modem since the router continues to work. In Windows 7 the network icon gets a little exclamation mark or something indicating "connected without internet access". Streaming continues just fine, but none of our machines or phones are able to get to the internet.
I have monkeyed with the router a bit a month or so ago, so that could be where something is a bit mucked but it's odd that this would be the only symptom of a problem.
Possibly also related is that this client was previously my media server on the few occasions when I used a media server. The library was copied from it to the new HTPC/server machine. The old copy of the library does still live on the client machine, but it is not being used since I'm connecting to the server.