Strange WoW Networking masquerading problem

[Taladar]

To Moderators: Please move this if you don't think the Networking Forum is the correct place for this

Last Thursday between 19:00 and 20:00 World of Warcraft stopped working for a lot of people (including me) on some Realms that worked for others.

The Problem looked like this:

Logon worked, you got as far as choosing a character and getting a loading screen for the world. Other players reported the
affected characters were online but the loading screen never vanished. This effect didn't affect all realms: same computer, same account, different realm server worked.

After some talk in the German as well as the English Forums (around 8 pages and after the weekend Blizzard even answered)
we (the players, Blizzard hadn't posted anything at that time) came to the conclusion that multiple ISPs (german AOL, german Telekom, QSC, British Telecom and a few others) were affected. All users with the problem did use Masquerading in some form, either with a "hardware" device or a Linux router. Some then tried a direct connection without the router which worked flawlessly.

I don't have that option but in the english forum thread I found someone who had used a program called SocksCap (basically forces a windows network app to use socks even if it doesn't support socks natively) and a Socks5 server to connect which worked too (I am using this myself, it works).

Now today some people found out that lowering the MTU on their hardware routers to 1400 solves the problem too (with Masquerading).

Now, the thing that looks strange to me:

Connection doesn't work completely with Masquerading and MTU set to default
(probably 1492 as most people here in germany use ADSL in a PPPoE configuration)
Connection does work completely without Masquerading and MTU set to default
Connection does work completely with Socks5 and MTU set to default (looks probably similar to the above but might have lower Path MTU, don't know enough about Socks)
Connection does work completely with Masquerading and MTU set to 1400

It only affects some realm servers, others work flawlessly.

It also affected lots of people at roughly the same time that had played the game in that configuration for months.

Blizzard says the problem is not their fault (as usual), the ISPs say the same (as usual).
Blizzard now recommends lowering the MTU when using Masquerading and sees the case as closed

I think that this is an ugly workaround and want to know the real reason behind this. I don't know enough about low-level networking code to come up with a plausible theory (at least not without further investigation which would require me to know the DNS names of the affected and unaffected realm servers; also using some networking tools on Blizzards servers might get my IP banned, I don't want that).

The Gentoo Networking Forum probably has more people who know about networking details than any other place I visit so I thought I should post this here and ask for theories what might have happened.