Roller shutters do not darken - really no reason for complaint?

That would drive me crazy too, why have expensive blinds if they don't darken properly. If it really can't be fixed, I would definitely add blackout curtains again, hopefully the combination will be enough for your little mouse. But it's still a lousy situation.

Make a recording outside in the daylight. I think they skimped on the "spacing" so that the area at the bottom where everything meets is not sufficient. The windowsill is generally too high, so the roller shutter doesn't close completely. However, independently of your roller shutter, I also believe that you can train your mouse to sleep in the daylight - after all, it will have to do that in [KiTa].

Ok, I'll take a picture from the outside tomorrow. How do you mean saved? How should it have been? Sleeping is such a topic with our little mouse. It's always been difficult. Long story ;-)

It looks like the rail profile does not go up to the windowsill.

I don't agree with that. Normally, the rails are open at the bottom, so even if they don't go up to the windowsill, the blind still goes all the way down and just kind of comes out of the rail.
I rather think that the seals on the blind rail (usually black, on both sides of the rail, so that the blind doesn’t rattle in the wind) are too tight. They practically clamp the slats at the bottom on the right side. On the left, the blind seems to go down further, right? Ergo, it pulls the blind crooked, down to the bottom on the right, hanging on the left.
A sharp utility knife blade would help, with which you simply cut away the bottom 5 cm of the black "blind rail seal."

No, going down to the left doesn't work either. I'll take more pictures tomorrow. Thank you for the many responses!

I do not share that opinion. Normally, the rails are open at the bottom, so even if they do not reach the windowsill, the blind goes all the way down

He confirms it, now the blind just has to be too narrow and light will come in.