Issue with kanban board

Hi,
I encouter error when try to create or edit a ticket in my Kanban ticket board. The steps to repeat the problem are as follow:
1. Open my Kanban ticket board.
2. Click 'add a ticket' or try to edit the existing ticket by clicking at the pencil icon.
3. Type some name if creating a new ticket.
4. Try to change the project in description tab.
5. Try to save the ticket.
6. Got disconnected notification.
This behavior is observed after I update to V12.1.0, at V12.0.1 I don't think I got this problem. Moreover, creating ticket directly in the ticket page do not cause this problem.
Thanks.