We need to keep running the command on subdirectories to find what is causing it. du -d1 -h /var/lib/
If you have plex installed, that is probably what is filling it.
I've got this far with reading over the contents of this thread, but I've got another culprit than Plex filling my system drive.
Problems may have started following one of the two following system updates, though I'm not sure:
Here is the output of the 'du' command.
I can't find info elsewhere on the forums about PCP or pmlogger, so I'm not sure whether/how to fix/remove.