The following error message sent via email by WordPress can sometimes be related to running an older version of WishList Member.
An excerpt from this type of message is included below for reference.
Error Message Excerpt:
Note: The file path, theme and referenced line in the message will be specific to your site.
If you receive a similar message, please check your WishList Member Dashboard and upgrade to the latest version if you are running an older version.
If you continue to have issues while running the latest version of WishList Member, please feel free to contact our Support Team.