Issue with Creases and Smudges during Duplex Printing on HP Color LaserJet Pro M452dn

[TheEleven]

Printer: HP Color LaserJet Pro M452dn

Computer: Windows 10
Connection: Printer and computer are connected via LAN.

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Problem:
There is no issue with single-sided printing. However, when duplex printing, after the sheet is fed for the second time, everything seems fine. But once it is output the second time, creases and smudges appear on the paper (see image).

For example, when printing 5 sheets duplex, the first sheet comes out fine, but starting from the second sheet, the problem occurs. The creases always appear in the same place, though sometimes they are more pronounced, sometimes less.

I have opened the printer doors to check for any toner residue or visible issues, but I couldn't find anything (unless I were to unscrew parts, which I am not willing to do yet). The problem appeared suddenly—the printer had no prior issues and is used almost daily.

Thank you for your help!
Since we are a school, I would be grateful if the printing of important documents can resume smoothly!

[Repairatrooper]

It could be simply poor quality media but, 99% of the time when I deal with a printer with those types of creasing issues it is a bad fuser. Unless there is something in the duplex paper path, which you states you have checked closely, a replacement fuser is going to be the solution.