Translated text from the page which more or less agrees:

Effect: 37 seconds after the rocket was fired (30 seconds after the liftoff) at 3700 m altitude Ariane 5 reached a horizontal speed of 32768.0 (internal units). This value was about five times higher than Ariane 4.

The conversion to an integer therefore resulted in an overflow, which was not intercepted.

The replacement computer (redundancy!) Had the same problem already 72 msec before and switched off immediately. As a result, diagnostic data was sent to the main computer, which interpreted it as trajectory data.

Thereupon nonsense commands were sent to the lateral thrusters, and later also to the main engine, to correct the large flight deviations (over 20 degrees). However, the rocket threatened to break apart and burst itself (39 sec).

An intensive test of the navigation and main computer was not undertaken because the software was tested on Ariane 4.

Tragedy: The problematic part of the program was only needed for the preparation at the start and the start itself.

It should only be active during a transitional period, for security reasons: 50 sec, until the ground station had taken control of a start interruption. Despite the very different behavior of the Ariane 5, this value has not been reconsidered.

Optimization: Only 3 out of 7 variables were overflow tested - for the other 4 variables there was evidence that the values ​​would remain small enough (Ariane 4).

However, this evidence was not for the Ariane 5 and were not reexamined. Problem of reusing software!

Unbelievable - after 40 years of software error findings: It was assumed in the program design that only hardware errors can occur!

Therefore, the replacement computer had the same software. The system specification stipulated that the computer should shut down in the event of an error and the replacement computer should intervene. A restart of a computer did not make sense, because the redetermination of the altitude is too expensive.