Reply To: popup in conflict by sending header and cookie

So I deactivated the contact form plugin to see what happens.

In admin popup preview there’s the following error permanetly present:

Warning: require(/home/megajob/public_html/wp-content/plugins/newsletter/popup/themes///theme.php) [function.require]: failed to open stream: No such file or directory in /home/megajob/public_html/wp-content/extensions/newsletter/popup/iframe.php on line 28

Fatal error: require() [function.require]: Failed opening required '/home/megajob/public_html/wp-content/plugins/newsletter/popup/themes///theme.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/megajob/public_html/wp-content/extensions/newsletter/popup/iframe.php on line 28

and in frontend is it still:

Warning: Cannot modify header information - headers already sent by (output started at /home/megajob/public_html/wp-content/extensions/newsletter/popup/iframe.php:1) in /home/megajob/public_html/wp-content/extensions/newsletter/popup/iframe.php on line 3

nad up from here there’s no conflict with another plugin. I can deactivate all other plugins. The error stays the same.