Hi,
I had the same issue. through empirical study (some might call trial and error) I figured your issue can be solved by clicking on the png button at the bottom left side of the guvnor process designer. This will generate the process picture and offer it to download afterwards. you can discard the download.
the image was created or updated anyway. afterwards build your guvnor package and the new image is deployed with it.
you have to repeat this step every time you make visual changes to the process or the diagram feature in the console will not work.
I also experienced that using swim lanes will mess up the diagram feature. the red arrows will be misplaced. my best guess is, that this is because the diagram feature is checking x,y position of the task objects they wand to point at. normally fine, but when the task box is within a swim lane box, the x,y is a relative value for the position within the swim lane box and not the ususal absolute value.
Hope you succeed.