Solid red is C4, as is show on autosport.
Autosport and Motorsport has the same exact feed. It's most likely the C4's, we'll just have to wait for Pirelli's morning update to know for sure.
Soymotor (spanish site) has people on site, and this is what they say (google translated):
... and he's out. So it was minor indeedFittingMechanics wrote: ↑27 Feb 2019, 11:54From SoyMotor - "We already know the cause of McLaren's red flag: loss of water pressure."
Team apparently told him to stop.