One of the two handbrakes is too tight and I can't find the barrel adjusters (as in this video: Adjust bike brakes, although his bike has V-brakes not mechanical Disk-brakes like me)?
Should I adjust the cable brake on the caliper (see picture below) to make the hand-brake less tight (Disk brake adjustment)?
Yes, in your picture of the caliper you can see the barrel adjuster at the left side. Usually when mechanics set up a cable brake, they’ll allow about one rotation of space for loosening, and the rest will be available to tighten the brake as the cable stretches and the pads wear. So, it’s possible that you may not have enough adjustment at the barrel to make it as loose as you want. If this is the case, you could loosen the allen bolt clamping the cable and let out a bit of slack there.
You could also adjust the position of the static pad, which is the one nearest the wheel. This is usually done with an allen key from the wheel side, turning it clockwise to move the pad inwards and anticlockwise to move it outwards. You don’t want to back it out too far, though, as this might cause the rotor to start rubbing the caliper body when you apply the brakes. You should be able to observe this easily enough.
So, I would start with the barrel adjuster, then if you don’t get enough slack from that, let a bit of cable through the pinch bolt.
caliper). I see something like an adjuster in the caliper. Can anybody post an answer so I can accept it?