I am getting this error after creating a few lists which happened without any issue. It seems to be related to another error where I try to delete some of the non default lists and it won't let me delete. The error message I get is: "You have chosen to delete your default list. The default list cannot be deleted." - But, I am pretty sure I haven't chosen the default list. The UI prevents you from choosing default list for deletion.
I am confused.
I think you have run up against a known issue with creating lists through the API. Our engineers are are that when creating lists through the API, more than one default list can be created if the <OptInDefault> element in the XML sumbitted with the call to create more than one of your lists contains "true." They are working to resolve the issue. For now, you can avoid it by adding some code to set the value the contents of this element to "false" in the XML when creating lists.
I apologize for any inconvenience or frustration the issue has caused you. Hopefully we can get it resolved soon, so an appropriate error is returned when attempting to create a second default list.