WordPress P+CDTR Not Working with Thematics

I have a WordPress site that I am building using Thematic and (trying to) P+CDTR (Text replacement).

It keeps failing with the error;

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/public_html/mysite/wp-content/themes/thematic/library/extensions/header-extensions.php:7) in /home/public_html/mysite/wp-content/themes/mysite/pcdtr/php/class.php on line 8

If I comment out th开发者_开发问答e seesion start in the P+CDTR class.php file I just get a white screen, no errors, no nothing.

Any help or ideas would be great.

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Edit /home/public_html/mysite/wp-content/themes/thematic/library/extensions/header-extensions.php and /home/public_html/mysite/wp-content/themes/mysite/pcdtr/php/class.php and make sure there aren't any blank lines or trailing white space before opening and closing PHP tags near the top and end of your documents.