1. The "Open this email in your web browser" link works only in some browsers.
Links from draft newsletters can only be opened by logged-in users. If the newsletter was not sent yet, you must be logged-in into the browser you're using to see the "Open this email in your web browser" link.
2. Users are getting the error message "Missing value for custom field" when subscribing.
If a custom field is set to be mandatory, it has to be included in all forms.