After upgrading to WordPress 3.5, I can no longer update plugins or save pages!
I have seen this happen for people who have their WordPress web hosting on Microsoft IIs servers. (most webhosts use Apache/Linux servers).
It turns out that WordPress 3.5 has a bug in it with regards to Microsoft IIs servers!
Don’t know what kind of server your site is on? Go to “What’s that site running?” and enter your site’s URL. The program will report back the type of server that your site is running on.
Unfortunately, you are between a rock and a hard place no matter what, as the fix needs to be applied manually, via FTP. For that there are two options…
Wait for WP version 3.5.1 to arrive (that looks to be a couple of weeks at least) and then, with FTP, remove and replace the WordPress 3.5 files and replace them with the WordPress 3.5.1 files.
1) Download and edit your wp-config.php, add the following line before “That’s all, stop editing”:
define( 'WP_TEMP_DIR', ABSPATH . 'wp-content/' );
2) Download and edit the file /wp-includes/class-http.php comment out lines 144 and 145, which look like the following:
if ( ! is_writable( dirname( $r['filename'] ) ) ) return new WP_Error( 'http_request_failed', __( 'Destination directory for file streaming does not exist or is not writable.' ) );
After applying one of the above fixes you should be able to add/upgrade your plugins and upgrade your themes, and the fix will allow you to use the auto upgrade feature when WordPress Version 3.5.1 is released.
My preference would be #1 above.
Here’s a link that explains the problem…