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I finally found the problem that was causing the ER65 code. On very very close inspection of the g-code I found that the layer was being duplicated at line 2201 for some reason. This put the print core at the same Z position on the layer it just finished thus not allowing any material to be extruded, and in combination with the flow sensor and the de-coupler on the material station it sensed that there was a problem. Disabling the flow sensor, or not using the material station prevented the layer error from being detected. This is why it always happened at the same spot no matter what bay, extruder or filament I used during testing.
This duplicated layer doesn't show up in preview in CURA. It's only because I have many years experience programing and running CNC machines and back plotting g-code that I found it. The back plot showed the Z not changing is what caught my eye.
I've not been able to reproduce the the code error in CURA 5.1
Andrew_W's post in Printing test file was marked as the answer