I just solved it. I don't know how sustainable my solution is.
I copied tar from another machine's /bin/tar, then i CHMOD'ed it to 755 on my destination machine's /bin/tar.
now reinstalling tar via apt-get works flawlessly:
root@devubuntu:/usr/bin# sudo chmod 755 /bin/tar
root@devubuntu:/usr/bin# sudo apt-get install --reinstall tar
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following packages were automatically installed and are no longer required:
xfonts-encodings php-console-table xfonts-utils libmcrypt4 libxfont1 xfonts-base php5-mcrypt
Use 'apt-get autoremove' to remove them.
0 upgraded, 0 newly installed, 1 reinstalled, 0 to remove and 7 not upgraded.
Need to get 0 B/218 kB of archives.
After this operation, 0 B of additional disk space will be used.
(Reading database ... 335218 files and directories currently installed.)
Preparing to replace tar 1.26-4ubuntu1 (using .../tar_1.26-4ubuntu1_amd64.deb) ...
Unpacking replacement tar ...
Processing triggers for man-db ...
Setting up tar (1.26-4ubuntu1) ...
when i use virtualmins "Re-Check Configuration" I now get:
All commands needed to create and restore backups are installed.
The selected package management and update systems are installed OK.
.. your system is ready for use by Virtualmin.
please notice, that
Note: root's PATH should usually contain /usr/local/sbin, /usr/sbin and /sbin.
is just a general information and NOT an actual error message! when I got this message, /usr/local/sbin, /usr/sbin and /sbin were already contained in my $PATH variable. so do not let it confuse you.