From the dashboard, on a petition I go to Petition settings > signatures > and click the big plus button.
A green flash box pops up saying
People are being added to "name of the list". Please wait. Job status: progress
but the green box then turns red and says
There was an error adding these people to your list.
I can add profiles individually to a list by clicking the plus sign by their name.
Update 10/31 - this has been fixed! The button will now properly add or remove your petition signatures from a list!
I am seeing this same issue. If this is affecting your workflow please let us know so we can prioritize. The more people who comment on this post the better!