If you are experiencing graphics issues it may be related to this bug
However please read the following before you assume that issue
ACPI PPC Probe failed. Starting version 219
These are two separate messages
ACPI PPC Probe failed.
Starting version 219
The first is related to a new ACPI interface PCC (Platform Communication Channel) https://github.com/torvalds/linux/blob/master/drivers/mailbox/pcc.c
Which is defined by UEFI http://www.uefi.org/ACPIv5.1
The second is related to systemd the program start process manager for Ubuntu 15.04 and above the current version of systemd is 219 so the message Starting version 219 is just stating the boot process has begun. Noted here: https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1432171
These messages are return prints of mutually exclusive events, they are not related to each other nor are they directly related to the Nvidia graphics issue.
You can see these messages without having any nvidia driver issues. Please do not assume you have a driver issue if you see these messages. Your PC may boot to the desktop after seeing one or both of them.
You may see this in your output of journalctl
Aug 16 23:10:55 x kernel: PCCT header not found.
Aug 16 23:10:55 x kernel: ACPI PCC probe failed.
These are the returns if the code does not find the ACPI interface which only some very new devices have or if it actually fails to read/write to the interface if it is present. The code was written for upstream releases. If you read the emails in the filed bugs below you will find information that supports my statements and you can read the return messages in the code itself in the github link above.
I hope this helps to clarify some things.