When the page is left idle for a long time and I start interacting with it, I see the "This ticket is locked" message which prompts me to refresh the page. When this happens I get the login screen with the "Session timed out due to inactivity" error message. However, it's impossible to login again after this - the login screen is just shown again each time with the same error message.
The only way to resolve it at this point is to clear the session cookies.
osTicket Core, v1.18.2