Until my content was approved, I figured it out by myself. ?♂️
For everyone with the same problem: Just put the map into ".../Command and Conquer Tiberian Sun/" and thats it. Don't touch the "/maps"-folder at all. Just put it into the main folder. I know it doesn't make sense, but it works! CNCnet Launcher will now recognize the map in its list and lets you choose it in skirmish or online lobby.
Have a good one and see you ingame!
JPlay