Q: no contacts after installing mountain lion

Same thing and the error message is :

[NSInvalidArgumentException] *** setObjectForKey: object cannot be nil (key: /3288520/carddavhome/card/)

I had the same problem.  Spent two hours on the phone with Apple support today and they were finally able to help me correct the problem. Had to go into the library and into the Application Support Folder and put the Addlress Book into the trash.  Re-start and it started working o.k.  If you are having this problem I would suggest you contact Apple support for detailed instructions on how to remedy the problem.

Thanks a lot. Problem solved 

I just wanted to add to the conversation that this was EXACTLY what happened to me after installing Mountain Lion. All my contacts were gone. It showed the Groups on the side of the address book after one day but the contacts never showed up. I tried unchecking the box for the contacts in system preferences for the iCloud and then rechecking it again.  That didn't help either. Came across this remedy up above about taking the Address Book from the Application Support Folder and putting it in the trash. Viola! It worked. However, I would add that I had to go into the iCloud pane and check the address book box since it then became unchecked. Once I did that, they ALL loaded back up and now they are finally there.

Thanks so much for this post. I had exactly the same problem, plus I couldn't use Mail — it was horribly slow and would freeze up. I finally figured out that the problem related to Contacts not having any database. So, I did as folks suggested here by trashing the "Address Book" folder, rebooting and rechecking Contacts in the iCloud System Preferences. ALL my problems went away — hooray!

Again, thanks!!!

I had the same thing happen with my MBP but not the iMac. I will call Apple Support in the a.m. as too late today. Thanks everyone.

I had the same issue & also called Apple Support. It took over an hour for the rep but using the same procedure, plus some, they all finally populated. I am a happy camper again. I didn't have this problem with my iMac yesterday, just the MbP today.

This worked immediately simply put the addres sbook in trash as stated above, go to preferances and re check contacts( As it is unchecked when you put the addreess book from application suport in the trash) The contacts load immediately AND the slow behavior of the mail box goes away too no beach ball( Obviusly from trying to syync with contacts) This solved all the problems I had with mountain lion

Thank you, thank you, thank you! I was having this exact same problem ever since upgrading to Mountain Lion.  It's been very frustrating waiting 5 or more minutes for an email address to pop up when starting to type an email and not having any contacts show up in Address Book.  I followed your instructions this morning and for the first time in months, my email is working properly and my address book is repopulated from iCloud.  Thank you so much!