Thanks everyone for your help, these are some interesting advices.

Found something out that is quite interesting. Noticed one morning that she had peed on the couch after she had used both her boxes during the night (peed into one them and pooped into the other one), so I decided to set up box #3 . She immediately walked into it and used it the same minute - and she has not used any other box since, she definitely loves the new one! I was thinking that her other boxes (even though they are regulary cleaned) might be too old and smelly for her little nose (one and two years old), so it may help to replace them by new boxes.

Don't know if this is already the solution for the problem, but at least one step into the right direction...

Kirsten