"Custom Signup" Lightbox Appears on Live Site Only (Includes Workaround)

Known Issue|We’re investigating this issue.
We're aware of the issue and are currently gathering more information regarding this matter.
Meanwhile, please use following workaround:
1. If Member Signup tab is missing, set a page permission to Members Only (so the member signup tab will be available).
2. Go to Member Signup Tab and Delete Custom Form. 
3. Save and Publish the Site.