Bug:

When you try to sleep in a bed on top of scaffolding the message "Bed is obstructed" is displayed.

Recreation: 

1) Place two scaffold next to each other and place a bed on it

2) Set time to night (Optional)

3) Try to sleep or set spawn at the bed

4) the message "Bed is obstructed" will be displayed

Note:

If this is not a bug, or already been reported, sorry for wasting your time Mojang.