thanks. I created it in Microsoft 3D Builder. But there it behaved like one object.
Before I bought Simplify3D, I printed it without problems in Cura.
Your tips in simplify 3d itself helped and worked!
Additionally I tried opening the STL in Microsoft 3D Builder again - and this time it showed me "there are errors" and I pressed the fix button and saved it. After loading in Simplify3D again, it looked, normal. So STL is fixed now... however it got broken