I'm running the Cyanogenmod version of Android on my Nexus 7 tablet (2012 wifi version). Once a week I've upgraded using the latest nightly CM12 build. Well this week it didn't go well.
On rebooting, I got the message "unfortunately Youtube has stopped" repeating in a loop immediately after I cancelled it. Various DuckDuckGo searches eventually turned up the answer. CM12 has changed to CM12,1 and my version of GApps needed updating too.
From the Cyanogenmod blog:
"CM 12.1 Nightlies
A week later than we anticipated, we’re ready to flip the switch for a multitude of devices to begin release of Android 5.1 based, 12.1 nightlies.
As we stated last week, these are safe to install on top of CM 12.0 official builds, with the caveat that you must manually update your third party add-on zips. For most of you, this boils down to finding an Android 5.1 compatible gapps if you want a working CM installation with Google apps that don’t crash.
Do recognize that these are the first releases, and as such they may have some cobwebs. Regression reports are being collected via our JIRA, so please do let us know if we broke something."
How to Flash Gapps via CWM Recovery: Go to “advanced” option and select “wipe dalvik cache”.
Go back to main menu in Recovery and install the GApps using “install zip from sdcard> choose zip from sdcard” option.
Go back to the main menu and select “reboot system now”.
All was well, and my impression is that the tablet is a bit zippier than before.