“The SPListItem being updated was not retrieved with all taxonomy fields.”
If you encounter the error above while trying to enable Enterprise Keywords field to a document library and adding a custom metadata field then the solution to this is the following(It’s a workaround, not sure the main reason why this happens, to me it happened in a record center):
Before you do anything related to the Enterprise Keywords field first add your custom metadata field THEN, only then do what you need to do and add the Enterprise Keywords functionality.
For some weird reason in my case no matter what I did I got the above error. Please notice that the above error can also happen if your metadata field does not have any connection to a term set. This problem was very annoying no matter what the situation was.