headers already sent by output started at uc_sagepayserver.pages.inc:273

On successful order there is the line:

echo _uc_sagepayserver_encode_response($data);

Which causes this error to be shown in the watchdog:

Cannot modify header information - headers already sent by (output started at sites/all/modules/uc_sagepayserver/uc_sagepayserver.pages.inc:273) in /includes/common.inc(1696) : eval()'d code on line 3.

If the line is commented out then numerous successful payment notifications are displayed in the watchdog yet the user is never redirected back to the site.