Thanks to selivem there is a solution for this available in the forum.
The root case is a link of
/var/tmp
to folder
/ftmp

The folder /ftmp is based on / per default, which is placed on /dev/md0 and usually rather full.

see https://sourceforge.net/apps/phpbb/freenas/viewtopic.php?f=55&t=2794 for more details. It helped with my embedded system as well.

Nevertheless, i would like to have an "official" solution as well, to be able to remove the shell command.

Maybe one can think about to link and map all tmp-folders ( /tmp, /var/tmp...) to a dedicated /dev/mdx device in the RAM.