Coherence computation

Yes, recently an updated in the connectivity functions was perfomed and the update in the tutorials will be there soon. Computation of coherence using Hilbert transform was before located within the Envelope correlation GUI. You can find information on how it is computed in this post: Lagged coherence - #3 by Raymundo.Cassani

What's the displayed error?

Check this post: Visualize Connectivity matrix using imagesc - #2 by Raymundo.Cassani

It seems you computed a coherence estimation for each time point of your data (thus the 8000 elements).