After upgrading WordPress to the current version, I could no longer automatically update plugins. This is the error message I got:
Downloading update from http://downloads.wordpress.org/plugin/xxx.zip.
Unpacking the update.
Warning: unlink(/…/wp-content/upgrade/xxx.zip) [function.unlink]: No such file or directory in /…/wp-admin/includes/class-wp-upgrader.php on line 146
Incompatible archive PCLZIP_ERR_MISSING_FILE (-4) : Missing archive file ‘/…/wp-content/upgrade/wp-syntax.0.9.8.zip’
After digging through the forums I found a solution to the plugin autoupdate problem in the WordPress Support forum.
Open the file wp-config.php in the root directory of your WordPress installation and remove the definition of the WP_TEMP_DIR variable, i.e. a line that looks like this
define('WP_TEMP_DIR', ABSPATH . "wp-content/upgrade");