Don't worry about it. I'm just glad you're willing to help!
In the meantime I resinstalled 3DGS from scratch on my PC. Did not change anything.
Tried your solution: yes, setting "Lit maps only" to "off" helps. Now I get ambient and sunlight. However, I only get grey ambient no matter which ambient color I choose.
And if I add a light in there, I get no sun at all.
Thanks for your help!