When clicking on the unsubscribe link in a campaign you receive a simple and great unsubscribe page and notice.
Unfortunately when clicking Re-Subscribe you encounter a 404 Error.
What do I need to do to correct this error?
You likely haven't upgraded to the latest version of the mailer plugin. You can do that by:
1) logging into your SFDC instance
2) go to this URL: https://login.salesforce.com/packaging/installPackage.apexp?p0=04td0000000AqtW
3) following the onscreen instructions from there to finish installing the package.