For future reference, a still photo is better than dropbox and a video (see Posting images on HB
for instructions). Here's another photo from your video:
It confirms my original suspicion: The steam boiler is full to the top with water; it's supposed to have a cushion of steam (see Heat Exchanger
in Espresso Machines 101
for details). Judging from the crusty external appearance of the boiler, my guess is that the water level sensor is caked with scale. The scale acts as an insulator, preventing it from detecting the water level. You could try draining the boiler and cleaning the water level sensor, but I'd bet the steam boiler is choking with scale and in desperate need of professional servicing.
To be clear, the safety valve is opening because it's designed to open at steam pressure (around 1.6 bar), but it's being pushed open by pump-driven water (around 9 bar).