Messages will not login; just sits and spins
My logic board was recently replaced in this Macbook and the vendor never serialized the newly installed logic board. I discovered this by going to "About This Mac" and seeing the serial number being reported as "Unknown".

The vendor corrected this actually applying a serial number to the logic board, which required specialized software that they had to boot off of from a DVD.

Once the serial number was applied then Messages worked just fine.