[Angel-Wings]

Says everything already

Check your phpinfo() screen for:

upload_tmp_dir

where temporary files are stored and if that directory is configured in "open_basedir"

If not - either change the upload_tmp_dir or set it to /tmp or maybe another directory where PHP can write files and change open_basedir as well to this directory while leaving the rest.
As example:

upload_tmp_dir = /var/db/apache/uploads
open_basedir = ".:/home/forum3at:/usr/lib/php:/usr/local/lib/php:/tmp:/var/db/apache/uploads/"