Hi Michael, thank you for your answer!
I used the Layout / Tree Tool; Layout Style "Directed" while allowing general graphs with straight-line connections. Your hint gave me the right direction: After changing the direction of one of the non-tree edges the algorithm found the right root node. Before this change a node with one incoming and four outgoing edges was chosen as root instead of the node with two outgoing edges / no incoming edge.
Best regards from Osnabrück, Philipp