That error is when a ticket is open on a terminal. Someone opens same ticket at same time on a different terminal and makes changes then closes ticket. Ticket logs would count as changes. If you do anything on ticket opened event etc.
A fix is to use the new feature in user roles that prevents this with the new mark ticket as open action and remove open ticket mark action.
If a ticket is marked as open then if the user role is set to not allow to display opened tickets the. It will not let that user open the ticket.