Android File Transfer doesn't like Samsung

Hey, Android Forum People.
I've been having a weird problem with the Android File Transfer software. I did a lot of research, my colleagues did too. No solution, none that work, anyway.
The exact problem:
My iMac can't connect to any Samsung device via AFT. Every other constellation works. iMac+Moto G3 works like a charm. Any other Computer, no matter if Win or OSX works fine with my S7 or Samsung Tab.
But my iMac+S7 Phones (I tried multiple different S7 devices) won't connect and it won't connect to my Samsung Tablet either.
I'm seriously confused.
USB debugging and everything is active, connection mode is MTP. Usually phones ask me for confirmation that I want to allow my iMac to access the phone. That dialogue simply doesn't show up, no matter what I do.
Also this used to work perfectly with this exact setup. Nothing changed, thing broke for no apparent reason.

If anyone has experience with this and can help me out, it would be greatly appreciated.
I've been sitting on this for days and I can't keep borrowing my colleague's laptop to transfer files to phones twenty times a day.

Thanks in advance.