Time Capsule disconnects internet when using web server through port forwarding

Question

My current config is: 2TB Time Capsule running 7.6.3, Mac Pro running Lion (10.7.5), MacBook Pro running Mountain Lion (10.8.3), and a couple of MacBook Pro Retinas running Mountain Lion. My current ISP is Comcast.
I have the Time Capsule set up to port forward port 80 to the Mac Pro for the purpose of running a web server. Most of the traffic is shuttling fairly large m4v files back and forth to work (work product from video editing work). I can reach the webserver on the Mac Pro just fine from the outside world, and can list files in the directory just fine. When I initiate a download (via right click on a link, "Save As..." or whatever), the Time Capsule reports that the Internet has been disconnected, and all machines inside my network lose access to the outside world. I have to either power cycle the Time Capsule, or disconnect/reconnect the Cat-6 cable between the cable modem and the Time Capsule to re-establish the connection. Interestingly, the download continues and completes on the other end without issue (so, the connection is actually still there!).
Before switching to Comcast, I was on AT&T Uverse - Uverse includes a router, so I had the TIme Capsule set up in Bridge Mode. That worked fine. I also have an older 500GB 1st generation Time Capsule. I set it up with the same config as my 2TB TC, and it shows the exact same behavior - internet disconnects upon initiating a file download through a forwarded port. I then hooked the Mac Pro up directly to the cable modem. I can initiate a download without issue, and I continue to stay connected to the Internet without problem. Called Comcast anyway - they report no errors on the line, but reset the whole device chain anyway. No change in behaviors.
I tried downgrading the Time Capsule software to 7.6.2 (the earliest version that came on it). No dice, same behavior.
So... I called Apple. Complete and utter FAIL. Apple Support reports that they have no training in the port forwarding features of the Time Capule or Airport products, and cannot help me resolve the issue because of that. So, I have a $300 router that apparently cannot perform a simple routing function (download through a forwarded port), that I cannot troubleshoot (because Airport software v7 is an epic fail in that regard), made by a company who cannot support it - and everything this $300 device is doing can be done by a $50 device with a $100 external hard drive?
Do I understand this situation correctly? What have I missed in trying to troubleshoot this thing?

Answer 1

Apple Support reports that they have no training in the port forwarding features of the Time Capule or Airport products, and cannot help me resolve the issue because of that. So, I have a $300 router that apparently cannot perform a simple routing function (download through a forwarded port), that I cannot troubleshoot (because Airport software v7 is an epic fail in that regard), made by a company who cannot support it - and everything this $300 device is doing can be done by a $50 device with a $100 external hard drive?
Do I understand this situation correctly? What have I missed in trying to troubleshoot this thing?
Excellent summary.. I think you have covered all the salient points most thoroughly.
Port forwarding on the TC I would put into the category of "epic fail"
I have a nasty feeling it just doesn't work in many cases. Clearly from your situation the port is forwarded correctly.. it is the actual download that is causing the device to crash.. which is probably not the port forwarding itself that is at fault but rather the external link and opening another port for the download to happen.
Many of the issues were caused by using icloud and btmm.. which would work a lot better btw. Although the hassle there is needing to offer that as a service to any Tom Dick or Harry.
Let me suggest a work around or other things to try.
I also have an older 500GB 1st generation Time Capsule. I set it up with the same config as my 2TB TC, and it shows the exact same behavior - internet disconnects upon initiating a file download through a forwarded port.
Use 5.6 utility.. more info below. And take the firmware of this unit (Gen1) back to 7.5.2 (before icloud mess ups in 7.6).
You can go back to 7.5 firmware also in Gen4 TC if it is more than about 14months old.. Simply hold down the option key when you select firmware and the old versions will show up. If your TC came with 7.6.1 (the 7.6.2 was withdrawn as it was even bigger failure than 7.6.3). then you cannot go back.
You might also find downloading via http is not as reliable as using FTP.. FTP is a better and more efficient protocol. It should be no problem to setup ftp server on the mac pro to allow download via ftp even with a browser.
But in the end.. your $50 router suggestion.. well perhaps a little more than that.. $80-100 will work very well.. also this router can be setup with a Hard Disk and offer the files to the internet via FTP.. without needing full network access to your internal network.
How to load 5.6 into ML.
1. Download 5.6 for Lion.
http://support.apple.com/kb/DL1482
Click to open the dmg but do not attempt to install the pkg.. it won't work anyway.
2. Download and install unpkg.
http://www.timdoug.com/unpkg/
Run unpkg on the desktop.. it is very simple.. drag the AirPortUtility56.pkg file over to unpkg.. and it will create a new directory of the same name on the desktop.. drill down.. applications utilities .. there lo and behold is Airport utility 5.6 .. drag it to your main utilities directory or just run it from current location.
You cannot uninstall 6.1 (now 6.2 if you updated) so don't try.. and you cannot or should not run them both at the same time.. so just ignore the toyland version.. the plastic hammer.. and start using 5.6.. a real tool.
For screen shots see this post.
https://discussions.apple.com/thread/4668746?tstart=0