Correct specification of TMPDIR Environment Variable in Debian

I run Urbackup Server Version 2.0.31 under Debian 3.16.36. After upgrade to Version 2.x the server uses /temp as temporary directory (Temporary file buffer enabled due to zfs dedup).
Have set TMPDIR in environment file and in the beginning of the urbackupsrv script in /etc/init.d with no effect (TMPDIR="/mnt/backup"). Can inquire TMPDIR and shows correct content. No error message or hint to temp directory in the urbackup-log. Target directory is ok and accessible.
Any proposal how to specifiy Envrionment Variable correct for urbackup - use ?
Temporary solution is remounting temp directory to target directory, but requires complete /temp to be redirected. Using the Environment Variable would be a much better solution.
Many Thanks

You can set it in the config file /etc/default/urbackupsrv. That one overwrites the environ variable (if present), so that could explain the problem.