I've run into this problem in Max 2010, wonder if anyone has a fix.
I'm unable to access map channel 2 in the UV editor. Switching to map channel 2, as circled in red, does nothing. In previous versions I get a pop-up asking if I want to perform this operation, then everything works as intended.
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not sure what drove them but way to many people including myself stumbled upon this and didn't knew further until after asking here or someone.
In TexTools I wrote a function that opens and closes the UV editor and makes the correct preparations in order to edit the specific channel - because it was way to confusing for me when working with multiple channels.
i expect it can be chalked in as user error, but im sure i lost some work. bah humbug.
*obviously not