The previous two questions regarding the ACPI error message do not have answers that fix my problem with the same error. I have just upgraded to 15.04 and cannot log in after a normal boot. The login screen shows briefly then reverts to a text only screen and alternates between the two screens on about a 1/2 second cadence. The laptop includes a nvidia graphics card. I am guessing that 219 is referring to an older version of the graphics controller but I could be mistaken.
I can only boot to recovery mode and use the system prompt from that mode. The syslog file did not reveal anything that I could recognize as useful.
What other log files might be informative?
How can I disable the nvidia driver so I can troubleshoot in normal operation?
Since my original post I have started ubuntu with the default video driver and purged nvidia drivers and installed nvidia-current-updates. Now when booting I still see the "ACPI PCC probe failed; starting version 219" message but I can log into the machine. Ubuntu reports an error in the Xorg area that I have yet to chase the details of. I am happy the machine is again usable but would like to pursue the source of the ACPI error. Any assistance would be appreciated.