Sorry I had not seen your post before today (I wish this site would use a forum application that notifies users...).. I have been struggling with the same error since upgrading my staging site to J!2.5.4 and K2v2.5.6. Did you find an answer to the error?

Hi Aaron,
I have just replied in your other post on the extrafield subject - hope it helps:

getk2.org/community/New-to-K2-Ask-here-first%21/11775-Extra-Field-array-broken-in-new-version

Regards
Mat
PS
I miss the notification system from the ning setup too! Please somebody add it to this new system ASAP!