Still not sure what is causing this, but all markets are open, as can be seen on the UTC Time.
At what time is it possible then to start / restart Z3?

It already has open positions on Z12, but Z12 startup / restart said the same, although it doesn't log out again, since other markets are open.

I would expect Z3 to give the warning, but stay logged in.

Actually not too long ago it did a successful trade... all other Zorros trade well, although Z12 e.g. gives same message, it at least then keeps adjusting the stops.