They just changed it this morning
Up to yesterday, it was sensor malfunction which was described in more detail as a snag of the sensor rod on the 2nd stage preventing separation and resulting in the first stage pivoting onto the 2nd stage fuel tank (with a following rupture).
As of this morning, it is lid failure on the separation engine. The first stage has small engines which guide them into the Korolev's cross positions after the explosive bolts have performed the initial separation.
That sounds more plausible than sensor failure - I am having some difficulty to imagine a sensor rod thick enough to snag something the size of the first stage against the forces exerted on it upon separation.
So actually, el reg info is a bit out of date (same as most other media).