Dell 3130CN has short blue horizontal lines on all prints

[torpedo]

Hello printer experts,

As per the title, these short (2mm to 4mm) horizontal lines on my Dell 3130CN are blue (only) and regularly spaced (29mm apart vertically ) on the left and right sides on all prints.

I did Contamination and Gradation tests but unfortunately the scans are too large to upload.

Any suggestions?

Many thanks.