Check for toner buildup on the upper fuser exit guide, see attached, can be hard to see and hard to believe just a little bit there will cause jamming.
I have lot of Jam 21 and they are all related to quality of paper , after long run machine is all hot and if paper is bad quality it fill jam at finger and cause jam 21 . You can lover Fuser temperature to see diference or put some quality paper inside just to see diference . I suggest navigator 80g/m2 , and You will see difference ........
I have also battled the 021 jams at heat pip turned out to be the one way gears that drive the belt, cleaning oneways helped but replacing resolved--Good Luck!!
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