I know this thread is old, but I just solved this and maybe my solution can help others. I also was not able to create any posts. My site had the list view working, but not the grid view, and every upload actually did go through although the upload feature would display the typical (media upload failed) error. I was able to view all of my photo uploads in /wp-content/uploads even after the library error.
I am new to PHP, and learning how to write my own scripts. I wrote a script to hide menu items/notices on the admin pages for my client, and I used:
add_action('admin_init' , 'makeDashboardPretty');
to hook into the admin pages to do this. This would run my script before anything on the admin page would load. One of the items I hid was my theme's menu item, which apparently needs to load in order to get the media library/posting working. I think the
echo ''; ?> css in my script would prevent my theme from enabling the media library/posting features.
I changed my admin hook to:
add_action('wp_after_admin_bar_render' , 'makeDashboardPretty');
which allowed my script to initialize after my theme's menu item did. This enabled me to hide the item and still get the theme working.