This isn’t related to site theme because it’s in the dashboard, and if that is indeed the case it shouldn’t be warning us that you need to assign a Activate and Register page when that is impossible to do.
The “hack” was just to visualise the button on the page so you can submit it.
Without the button you couldn’t change the Members, Activity Streams or User Groups selections on the same form.
I had the same problem, the button is there but hidden by its class “clear” (for no reason I can work out) I solved the issue by using chrome inspecting the page elements and finding the submit paragraph class.
Then I used Chrome to edit the temporary status of the class by removing the “clear” option. That then revealed the button and I could proceed with the save.
Still no idea what happened there. But I hope this works for you.
