I don't know if this is something CloudFlare is doing, or something that can be fixed...I know there are other issues being worked on currently, maybe it's related?
For about a week now, I've been unable to browse the forum on my New 3DS (I haven't tested an "old" 3DS), I just get a white screen with "Invalid data supplied" in the upper-left corner. Today I tried, and I actually got the login page, and managed to log in, and when I clicked on "View Your Posts", it took me back to the error page.
I can't imagine there are many others in my boat, but I thought I would mention it anyway.
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I'll brave the storm to come, for it surely looks like rain...