@picoflug and @Felix
This changed in 4.0 because the parameter H_LAND_COL_MIN was removed. This parameter is used to limit how low the collective can go in ALTHOLD, LOITER, POSHOLD, and AUTO flight modes. basically any mode where the autopilot is controlling collective. The autopilot also uses it to judge when it is landed. So it is VERY IMPORTANT to set it correctly. If it is set improperly your helicopter will crash in while doing auto landings.
This function is now handled by H_COL_MID. it cleans up the number of required parameters. So please be sure H_COL_MID is set correctly. I have tried to make this as clear as possible in the release notes and on this site.