Printer printing smudged line down the length of the paper

[missuseff]

We are using an HP LaserJet Pro 200 color Printer M251nw.

It has been printing for a while now with a smudged line down the paper, about a 1/3 of the way from the right margin.

We have been replacing toner cartridges many times since the problem started, but the problem persists.

We have removed cartridges and its tray to clean off the transfer belt but we still have the problem.

We have cleaned the roller in the back panel and still have the problem.

We have run the cleaning page multiple times, but the problem persists, and the line doesn't seem to get lighter or thinner.

Is there anywhere else we can clean to get rid of the line?  Or anything else we can do?

Thank you!

demo page with smudged lineclose up of smudged lineclose up of smudged linesmudged line down the color quality page

[Valsimot]

Looks like a transfer belt the black plastic foil under toner cartridges, not sure can it be cleaned, probably a new belt will be needed.

[missuseff]

Thank you for your response!