Logged out when submitting ticket updates

We've got a weird issue with our osTicket install.

When logged in as an agent, and going in to update a ticket (whether that's adding new information, or closing it), on pressing submit it returns us to the login screen, saying authentication required. However the update has gone through and the ticket is updated.

We can however update system settings etc in the admin panel without being logged out, and users have no issue, it's literally just on the agent panel.

We're a bit stumped on this one, could anyone kindly offer their assistance?

I've attached a screenshot of the system information page.

Thank you in advance.

Ben