Problem with temporary folder

You need to contact your hosting provider.

We use standard PHP function to get a temporary file:
https://php.net/manual/en/function.tmpfile.php

This function failed on your server. Your hosting provider must fix this.

I am administrator of my server. What can I do to make it work?

Go through your php settings and make sure it is looking for the temporary folder in the correct place and it has rights to write to that place.