Errors when using PSA updater Reloaded 7.5.4

[Fich]

Hello
Can someone please help me solving this problem

I'm running PSA Reloaded 7.5.4 on a Debian Sarge installation and when i use the buildin updater from PSA control panel, I receive the following errormail:

(First part removed to shorten length of text)
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The following NEW packages will be installed:
psa-hotfix8
The following packages will be upgraded:
psa-spamassassin
1 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 592kB of archives.
After unpacking 1217kB of additional disk space will be used.
Get:1 https://autoinstall.plesk.com PSA_7.5.4/ psa-hotfix8 7.5.4-debian3.1.build75060618.13 [558kB]
Get:2 https://autoinstall.plesk.com PSA_7.5.4/ psa-spamassassin 7.5.4-debian3.1.build75060618.13 [34.7kB] Fetched 592kB in 1s (358kB/s)
dpkg: `ldconfig' not found on PATH.
dpkg: `start-stop-daemon' not found on PATH.
dpkg: `install-info' not found on PATH.
dpkg: `update-rc.d' not found on PATH.
dpkg: 4 expected program(s) not found on PATH.
NB: root's PATH should usually contain /usr/local/sbin, /usr/sbin and /sbin.
E: Sub-process /usr/bin/dpkg returned an error code (2)
ERROR: Install command execution failed

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Plesk autoinstaller.

Kind regards
Fich