The interessting part of the Error message is:
```
Failed to restart multipath-tools.service: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists
See system logs and 'systemctl status multipath-tools.service' for details.
invoke-rc.d: initscript multipath-tools, action "restart" failed.
○ multipath-tools.service
     Loaded: error (Reason: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists)
     Active: inactive (dead)
Warning: The unit file, source configuration file or drop-ins of multipath-tools.service changed on disk. Run 'systemctl daemon-reload' to reload units.
```

After reboot this is still the case:
```
root@resolver-test:~# systemctl status multipath-tools.service
Warning: The unit file, source configuration file or drop-ins of multipath-tools.service changed on disk. Run 'systemctl daemon-reload' to reload units.
○ multipath-tools.service
     Loaded: error (Reason: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists)
     Active: inactive (dead)
root@resolver-test:~# systemctl restart multipath-tools.service
Failed to restart multipath-tools.service: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists
See system logs and 'systemctl status multipath-tools.service' for details.
root@resolver-test:~# journalctl -u multipath-tools.service
-- No entries --
root@resolver-test:~# systemctl status multipath-tools.service
Warning: The unit file, source configuration file or drop-ins of multipath-tools.service changed on disk. Run 'systemctl daemon-reload' to reload units.
○ multipath-tools.service
     Loaded: error (Reason: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists)
     Active: inactive (dead)
root@resolver-test:~# systemctl daemon-reload
root@resolver-test:~# systemctl restart multipath-tools.service
Failed to restart multipath-tools.service: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists
See system logs and 'systemctl status multipath-tools.service' for details.
root@resolver-test:~# systemctl status multipath-tools.service
Warning: The unit file, source configuration file or drop-ins of multipath-tools.service changed on disk. Run 'systemctl daemon-reload' to reload units.
○ multipath-tools.service
     Loaded: error (Reason: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists)
     Active: inactive (dead)

```

multipath-tools is depended by ubuntu-server but not really used.

Even purging it fails:

```
root@resolver-test:~# aptitude purge multipath-tools
The following packages will be REMOVED:  
  multipath-tools{p} 
0 packages upgraded, 0 newly installed, 1 to remove and 0 not upgraded.
Need to get 0 B of archives. After unpacking 1,256 kB will be freed.
Do you want to continue? [Y/n/?] y
(Reading database ... 182822 files and directories currently installed.)
Removing multipath-tools (0.8.8-1ubuntu1) ...
Failed to stop multipath-tools.service: Unit multipath-tools.service not loaded.
invoke-rc.d: initscript multipath-tools, action "stop" failed.
dpkg: error processing package multipath-tools (--remove):
 installed multipath-tools package pre-removal script subprocess returned error exit status 1
dpkg: too many errors, stopping
multipathd.socket is a disabled or a static unit, not starting it.
Failed to start multipath-tools.service: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists
See system logs and 'systemctl status multipath-tools.service' for details.
invoke-rc.d: initscript multipath-tools, action "start" failed.
○ multipath-tools.service
     Loaded: error (Reason: Unit multipath-tools.service failed to load properly, please adjust/correct and reload service manager: File exists)
     Active: inactive (dead)
Warning: The unit file, source configuration file or drop-ins of multipath-tools.service changed on disk. Run 'systemctl daemon-reload' to reload units.
dpkg: error while cleaning up:
 installed multipath-tools package post-installation script subprocess returned error exit status 1
Errors were encountered while processing:
 multipath-tools
Processing was halted because there were too many errors.
E: Sub-process /usr/bin/dpkg returned an error code (1)
Processing triggers for libc-bin (2.35-0ubuntu3) ...
```

Commenting out 
```
if [ -z "${DPKG_ROOT:-}" ] && [ "$1" = remove ] && [ -x "/etc/init.d/multipath-tools" ] ; then
       invoke-rc.d multipath-tools stop || exit 1
fi
```
in /var/lib/dpkg/info/multipath-tools.prerm, allows me to purge `multipath-tools` and install `ubuntu-server` including the `multipath-tools` again afterwards.