I succeeded to reproduce the issue which you described. This is a bug which I have logged. It will be fixed for the next version of Sitefinity 6.1. For the time being I can suggest a work around.
Please delete your mailing list using the following method:
public void DeleteMailingList(Guid id)
NewslettersManager manager = NewslettersManager.GetManager();
if (manager.GetMailingLists().Where(ml => ml.Id == id).Count() > 0)
The id of your mailing list you can find in sf_lst table in the database.
Once you delete your mailing list, you will be able to enter the Email Campaigns screen and modify your campaign to use another list.
Please let me know if you have any questions and apologies for the inconvenience.
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