Unable to create a solid

 From:  Michael Gibson
11430.3 In reply to 11430.1 
Hi Gunter, or another way is join up these 3 segments at each corner:



Select the 4 corner curves:



Run Construct > Loft and set loft options for "Loft style = Straight" and turn on the "Closed" checkbox:





Then the top is planar so you can finish it by selecting the object and run Construct > Planar, it will fill in the top.

- Michael