Problem placing stairs between basement and ground floor

In one of my builds I've got the problem that the staircase between 1th floor and basement can't be used by sims. This is a staircase placed underneath the staircase to the 2nd floor and has 2 landings. There are no objects blocking the way. I placed this with move objects on. Every time I want my sim to go down the stairs it will tell me something is blocking their way. My build is named Church of the fledermaus and my ID is RikkieMoSims