Seeing the log in real time
I've found that during shutdown there is usually an Ubuntu logo and flashing lights, which is shown instead of a log of the shutdown process. If there are errors, then they're kind of shown, but messily. However, whilst shutting down, if I press the Windows key and
r (Metar), then I get to see the success and failure of the system services, as they occur. So then I know what exactly is broken. No idea whether this keyboard shortcut is specific to my Kubuntu setup or what; I didn't add it.. One of those I found by accident, somehow...
Viewing the logs after rebooting
When the system is rebooted, the error messages should be saved into a logfile. Which logfile depends on which service is broken/misconfigured. The relevant log will almost definitely be in
/var/log/ (or a subdirectory thereof).
find have been the only programs I've needed for finding error messages in the logs...
Once you've found the error and the service that caused it, then you shouldn't need to reboot to test the new configuration; just restart the service.. Hopefully you can test the fixed configuration with a command like:
sudo service <service name> restart