Using Tuleap 8.14
When I open a bug from the Kanban, it pops-up a dialog to edit the bug. If I try to add myself in CC and save, I get the error: "Internal Server Error: Fields are not valid". I attached a screen shot to make sure this is clear.
If you want to try this yourself, the Kanban page is here:
https://tuleap.eclipse.org/plugins/agiledashboard/?group_id=102&action=showKanban&id=6#/kanban
For example, I opened bug #243 and added myself in CC then clicked save.
I think some people for Enalean have an account there so it should be possible to test this :) If not, let me know.
BTW, if I open the same bug in a separate page (middle-click), then this works correctly.