The tech-demo phase of Ariane 6 is still in progress but has shown an unexpected result which will only affect the end of the mission.
On the webcast, they stated that the Auxiliary Propulsion Unit (APU) switched off shortly after it it powered up after SECO-2. Without the APU, they have no way to settle the propellants in the tanks, and thus can’t reignite the upper stage for the final burn.
From the live camera feed, it looks as though the upper stage was spinning, so there may have been some loss of control as well.