PX4-Autopilot: Multi-Vehicle Simulation with Gazebo not working: single_vehicle_spawn.launch returns error

There seems to be a problem with the launch files and the models of the UAVs. When I follow the example in the developer guide (https://dev.px4.io/en/simulation/multi-vehicle-simulation.html) the launch (roslaunch px4 multi_uav_mavros_sitl.launch) in step 3 fails with following result:

ERROR [Unknown] Unknown/unhandled parameter: -s
Restoring terminal
process[uav2/iris_2_spawn-8]: started with pid [17474]
[/uav1/mavros:351]: Plugin ftp initialized
[/uav1/mavros:301]: Plugin global_position loaded
process[uav2/mavros-9]: started with pid [17475]
[uav2/sitl_2-7] process has died [pid 17473, exit code 1, cmd <my home folder>/Workspaces/px4/Firmware/build/posix_sitl_default/px4 <my home folder>/Workspaces/px4/Firmware/ROMFS/px4fmu_common -s etc/init.d-posix/rcS -i 2 -d __name:=sitl_2 __log:=<my home folder>/.ros/log/64a1c728-b272-11e8-82c6-3413e83b56c3/uav2-sitl_2-7.log].
log file: <my home folder>/.ros/log/64a1c728-b272-11e8-82c6-3413e83b56c3/uav2-sitl_2-7*.log

This problem originates in the single_vehicle_spawn.launch. If I remove the arg -s […] part, I get the error for the arg -i […]. If I also remove that part, the launch proceeds without errors but MAVROS cannot connect to the PX4 SITL flight controller.

This was working properly with v1.8.0 but not with the latest version of the master branch.