AEG Lavamat 74800: Door doesn't open, i.e. opens with long delay

After a trouble free period of almost two years, recently the washing machine developed the habit to keep the front loading door locked for at least 10 minutes after the washing cycle has finished. This wasn't the case before. The manual mentions such a problem IF the electricity fails, but this has not happened. The delayed opening of the door is a permanent problem.

We brought the machine from Germany to NZ and we have no customer service around. There is a minor difference in the voltage: 240 Volt in New Zealand, vs. 230 Volt in Germany, but the Hertz is exactly the same.

Any suggestions how to fix this problem? May be someone could explain how the door lock operates and which conditions could cause this problem (after it was fine for almost 2 years).

Thanks!