We bought our first "Jöö bunny" yesterday.
When we tried it out, it lit up wonderfully.
But when we wanted to hand it over to the kiddy, it stopped working.
As you can't exchange it at Migros on a Sunday, I checked it out and discovered the following:

Solution:
There are 3 LR44 button cells in the plastic box that can be removed from the back of the bunny.
Unfortunately, the battery contacts are very bad, you have to "bend" them like this.
The contact from the bottom has to be bent up slightly so that it protrudes over the plastic of the base and can thus make contact.
The side contact must also be bent out to the side, from below. The contact spring is fixed at the top and can be bent out at the bottom.
This is best done with a tiny watchmaker's screwdriver.
Once this is done, the rabbit will work perfectly again.
Then you have to put the plastic box back into the rabbit (have fun).
Why the plastic box is so big that you can hardly get it back into the rabbit is not clear to me,
because the box is 90% empty.

Since the battery contacts are not made of spring steel, you probably have to repeat the procedure from time to time.

Hope this helps others with babies in the dark ...
Steff