Pale horizontal line with small dots above it

Hey there,

I have a problem with the WorkCentre 7835, it prints on every single page a pale horizontal line with 5 or 6 small dots above the line. Here is an example picture:

I have already tried to clean the IBT belt but the problem persist. If R1 and R3 are switched, the dots become magenta and the line more visible. Here is a picture:

Can someone tell me where my problem is and how can I resolve it, please?

Thank you!