It looks like one of your scripts are trying to declare the mailer class twice. You have to check your modification and if you cannot find anything compare your scripts to the untouched sources in the archive from Groupee. You can use Beyond Compare from scootersoft to do this.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.