I´ve had the same problem for the last few days as well, being unable to access any Arlo cams through the ST app.
Motion detections are logged, but not able to se live view, battery level (reported as 0%) or anything else.
My hunch is that a Arlo / ST “integration agreement” is up for renewal 
Or maybe it is related to the announced changes ST is currently doing (for instance in terms of a “wind-down” of cloud-based device status monitoring);