When I go to my Extension Manager Warning section I'm seeing this error:

"The PHP temporary directory is not set. The PHP temporary directory is the directory that PHP uses to store an uploaded file before Joomla! can access this file. Whilst the directory not being set isn't always a problem, if you are having issues with manifest files not being detected or uploaded files not being detected, setting this in your php.ini file might fix the issue."

The solutions were taken from this forum thread

In the configuration.php file there's an attribute called $tmp_path. Set that to the temporary path that Joomla should use.

It's usually a subdirectory of the root called /tmp.

And I'd say that the problem is more likely the write permissions to that directory rather than the directory not existing.

For Example

public $tmp_path = '/usr/local/pem/vhosts/1xxx49/webspace/httpdocs/tmp';

It might also be that this tmp folder is not set in your php.ini file

or

It's actually related to the open_basedir config of the web server. if you have access to the root of your ftp and php is run as FastCGI you can add your custom php.ini to your virtual host which should contain the following information. I'm using parallels so this might be different for plesk.{code}open_basedir = /var/www/vhosts/domain-name/httpdocs:/tmp/{/code}