I'm just wondering if there is a handler that I'm not seeing in the code that replaces commas in group names with an escaped comma as per the api documentation. I haven't seen it anywhere but that doesn't necessarily mean it doesn't exist.

Comments

I had a problem which I suspect is related - one of my interest groups had double quotes in it.

Whenever I tried to sign up for that interest group, the form wouldn't let me ("An illegal choice has been detected").

When I changed the name of the interest group, it was fine.

Category:support» bug

Comma's aren't escaped in d7 version either. This is making campaignSegmentTest fail. At least it does for me, should probably be fixed in the module/API.
Then again, campaignSegmentTest isn't called anywhere by the module. But I am using it in a custom module. So I just cleaned up the input before calling that function.

Priority:Normal» Major

This is bad. The module implodes your interest groups adding commas, then sends it to mailchimp API. So.. mailchimp API is expecting a comma delineated list of values?

So you cannot have commas in your "interest group" fields...

Well, you can have comma's, but they do need to be escaped. See #1192556: Escape commas in interest group name for inspiration/explanation. I used something like str_replace(',', '\,', $group); in my own module.