I've never used adaptive cruise but my car used to tell me I was tailgating almost any time it could detect a car in front of me, seemingly regardless of distance or speed. I figured out how to turn off the warning and haven't looked back.
The adaptive cruise may need to be re-calibrated. If you've ever unplugged it (say, to remove your bumper or anything like that) it's probably messed up now.
If you ever need to switch from ACC to CCS, see my post here
ACC to CCS procedure