Emulator

Game Name?
Super Smash Bros. Brawl

Game ID?
RSBE01

What's the problem? Describe what went wrong in few words.
In-game error 20100

What did you expect to happen instead?
No errors

What steps will reproduce the problem?

1. Try to connect to Nintendo WFC, pretty straightforward...

Dolphin 3.5 and 3.5-367 are old versions of Dolphin that have
known issues and bugs, so don't report issues about them and test the
latest Dolphin version first.
Which versions of Dolphin did you test on?
4.0-1001

Does using an older version of Dolphin solve your issue? If yes, which
versions of Dolphin used to work?
Yes, 4.0-996

Are you using the 32 or the 64 bit version of Dolphin?
64-bit

Is there any other relevant information? (e.g. logs, screenshots,
configuration files)
FYI you will gonna need a Dolphin with required files needed to connect to Nintendo WFC. You can read instruction at https://dolphin-emu.org/docs/guides/wii-network-guide/

I don't know (care) about other Nintendo WFC supported Wii games. Only Brawl [RSBE01].