What should I do if a roll of paper jams in the HP DesignJet T520?

Roll jam troubleshooting is addressed in Chapter 11 of the manual under the section 'Roll has jammed.' You should follow the instructions provided in that section to clear the jam safely. Additionally, the manual notes that if you have an unexpected problem at any stage of the paper loading process, you should refer to the 'Paper cannot be loaded successfully' section on page 84. (Page 87)