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

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.