If you use a certain special character in a class's Owner or Comment field, you may not see the class tree or the reports in the Traffic Management page in the Sky UI.

The special character that can cause this issue is | (the vertical bar character, sometimes used to mean 'OR').

The Comment and Owner fields, available when editing a class, allow the | character to be entered, causing this issue.

Edit the class and remove the | character from the Comment or Owner field; apply changes.