Help: Sunnto D6i and Dive Plan mode

[charlier]

My backup computer (Sunnto D6i) shows NA , desat 0:00 on "Plan no dec" mode. The battery is OK. All else works fine, from Nitrox mode to Dive History. I mostly use the D6i on gauge mode, but clearly it was on Dive-Nitro mode during the last dive. After 500+hours as backup computer, I would have thought this might have happened before.

Helpful suggestions welcome and please do not suggest that I replace the computer with a more reliable brand.