I edited my map in several places, compiled it, typed in the console "map <mymap>" the loading screen was there for one second and then this showed up in the console:

] map 04Water
maxplayers set to 1
---- Host_NewGame ----
Network: IP 192.168.2.108, mode SP, dedicated No, ports 0 SV / 0 CL
Opened Steam Socket NS_SERVER ( socket 3588 )
Opened Steam Socket NS_CLIENT ( socket 3156 )
Network: IP 192.168.2.108, mode MP, dedicated No, ports 27015 SV / 27005 CL
Host_EndGame: Map coordinate extents are too large!!
Check for errors!

But my map isnt big at all, rather relative small. And I returned to the main menu after that. What the hell does that mean? Cant find it in the wiki or here in the forums.

Problem solved. Restarted Hammer and it found a not solid object, which was removed. Dont know what that was, but its working again now.

This can happen when you edit a brush using the vertex tool or by carving out weird shapes using the carve tool. Avoid using these tools unless you really now what you're doing. I made the mistake of using the vertex tool a lot and I ended up with a similar error but I wasn't fortunate enough to be able to recover my map in my case. Use clipping tool to avoid this in the future.