Omeka has encountered an error

Zend_Controller_Response_Exception

Cannot send headers; headers already sent in /home/digital8/public_html/plugins/GuestUser/controllers/UserController.php, line 8

Zend_Controller_Response_Exception: Cannot send headers; headers already sent in /home/digital8/public_html/plugins/GuestUser/controllers/UserController.php, line 8 in /home/digital8/public_html/application/libraries/Zend/Controller/Response/Abstract.php:323
Stack trace:
#0 /home/digital8/public_html/application/libraries/Zend/Controller/Response/Abstract.php(341): Zend_Controller_Response_Abstract->canSendHeaders(true)
#1 /home/digital8/public_html/application/libraries/Zend/Controller/Response/Abstract.php(768): Zend_Controller_Response_Abstract->sendHeaders()
#2 /home/digital8/public_html/application/libraries/Zend/Controller/Front.php(992): Zend_Controller_Response_Abstract->sendResponse()
#3 /home/digital8/public_html/application/libraries/Zend/Application/Bootstrap/Bootstrap.php(106): Zend_Controller_Front->dispatch()
#4 /home/digital8/public_html/application/libraries/Zend/Application.php(384): Zend_Application_Bootstrap_Bootstrap->run()
#5 /home/digital8/public_html/application/libraries/Omeka/Application.php(73): Zend_Application->run()
#6 /home/digital8/public_html/index.php(23): Omeka_Application->run()
#7 {main}