To fix this issue, go into the workstep and amend the 'Note' section where you found the invalid character.
Follow these steps to amend the case note:
📌Note: First, try it in a non-live environment to see if it fixes the issue.
Find the person.
On the personal summary screen, click on Case Notes.
Locate the affected case note.
Click the Edit icon.
Remove the invalid character from wherever field it's present.
Click Save.