The problem is that I seem to have a rostopic name clash between the simulation and the real system. I'm trying to put the real system and the gazebo simulation in their own namespaces. It works for the real system but the simulation does not start correctly when put in a namespace. It seems like it still tries to reach /gazebo/spawn_urdf_model service without the namespace prefix. The other services seemingly correctly have the namespace prefix.

It seems like it still tries to reach /gazebo/spawn_urdf_model service without the namespace prefix.

this is a very specific problem that would've been good to mention in your OP.

In any case: according to the code spawn_model should be able to deal with namespaces. Could you perhaps show a snippet of your .launch file(s) that show (roughly) how you start your simulation?

I'm sorry, yes, I should have mentioned it. I'm using the ur_gazebo package, completely unmodified. The only thing I'm doing is launching the simulation by ROS_NAMESPACE=simulation roslaunch ur_gazebo ur5.launch. So from my point of view nothing special.

I'd try with a wrapper-.launch file that uses a group with a ns attribute and includes the launch files that you want to start in a different namespace. It might not change things, but it's a slightly more reproducable -- and shareable -- setup.

Alright, I'll give it a shot. For the acute situation though I have now performed the moves on the real robot, drastically slowed down in order to be able to react in the case of an imminent collision. If a run turned out working without collision, I executed the trajectory at full speed. A bit of a risk, but no issues were had. Even without using the simulation.