Challenge "Extra Content" Error?

I can download and fight against almost all challenges without problems. And, I can issue them apparently without problems. But when I try to test out the challenges I’ve issued, it gives me an error saying that “this player has extra content…” and takes me to the web site, which says that this was likely modded content. Since there are always 0 attempts on my challenges, I assume other people get this error as well (or I have cooties and no one wants to play with me). I have not modded the game in any way (unless deleting the tutorial ships counts), and, anyway, shouldn’t I at least be able to run my own challenges? Running on a Mac, with all available dlc.

I’m not sure if this is a bug, or something I’m doing wrong. Thanks for any help. GSB is wonderful.