Linux Mint ADB sees my phone as 'offline' & how to 'untrust' computer

After connecting the phone via usb cable, "adb devices" command lists :

$ adb devices
List of devices attached
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
FA3BSWG00401 offline

1) First time it asked whether to trust this computer and I checked 'always trust this computer'. Is there a way to untrust a specific computer / make it ask again ?
2) plugging the cable to my older SGS3 works fine :
$ adb devices
List of devices attached
4df1d4650790afdf device

Has anybody encountered this before ?

Additional info :
$ adb version
Android Debug Bridge version 1.0.32
Revision 57224c5cff69-android

I'm using Linux Mint 17.2, NuSense 6 (Lolipop)