I am having the same weird output issues. This client has had 180 submissions and I need to export!!!
I replaced the Admin.php with the one you provided above and no change. Please help, I sincerely appreciate it!
Output exaample below
`<br />
<b>Fatal error</b>: Uncaught exception ‘Exception’ with message ‘Serialization of ‘Closure’ is not allowed’ in /home/truefoun/public_html/wp-includes/class-wp-query.php:1043
Stack trace:
#0 [internal function]: SplDoublyLinkedList->serialize()
#1 /home/truefoun/public_html/wp-includes/class-wp-query.php(1043): serialize(Array)
#2 /home/truefoun/public_html/wp-includes/class-wp-query.php(1668): WP_Query->parse_query()
#3 /home/truefoun/public_html/wp-includes/class-wp-query.php(3238): WP_Query->get_posts()
#4 /home/truefoun/public_html/wp-includes/post.php(1689): WP_Query->query(Array)
#5 /home/truefoun/public_html/wp-content/plugins/contact-form-submissions/Admin.php(449): get_posts(Array)
#6 [internal function]: WPCF7SAdmin->export_request(Object(WP))
#7 /home/truefoun/public_html/wp-includes/class-wp-hook.php(298): call_user_func_array(Array Array)
#8 /home/truefoun/public_html/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters(” Array)
#9 /home/truefoun/public_html/wp-includes/plugin.php(515): WP_Hook->do_action(Ar in <b>/home/truefoun/public_html/wp-includes/class-wp-query.php</b> on line <b>1043</b><br />