Rear wheel not centred, presses against one of the brake pads (Rohloff hub, Magura rim brakes) Upon replacing my rear brake pads, I noticed that my rear wheel is not properly centered.
The consequence is that the wheel is pressed against one of the brake pads, as can be seen in the pictures below.
I have a Rohloff hub where I can pull the dropout+wheel back in order to tighten the chain, but I don't know how to center the wheel horizontally.
In the final photograph (number 4) I have marked with A and B the dropout.
Normally, I put in the wheel by turning the bicycle upside down and then let the wheel into the dropout, going "all the way".
But If i do so, it results in the rim being pushed against the brake asymmetrically.
If I slightly raise at A and then close the quick release lever, the wheel is free from the brake, but it is not very stable, I think it's not the normal situation.

The wheel appears centered if I test the distance to the axle when putting it on the floor:




Dropout looks like this:



My question: how do I keep the rear wheel centered in the axle and straight for my bicycle (Rohloff hub with longitudinally movable dropout, Magura HS33 rim brakes)?









Edit:

It appears the wheel is straight, but it was not centered. I have those pads on both sides, and one side was pulled back much more than the other. I determine this based on the length of the visible part of the thread. I fixed it now, but I think the reason it keeps going bad regularly is because one of the screws is missing:




See the void in the second picture...

I remember that I unsuccessfully hunted for such a screw, but then concluded that it's not essential and continued cycling. But I can't anywhere in this area find a screw with an inbus-head and the right diameter. I used a screw with another head, but couldn't tighten it very well and — as I found out now — it fell out again...
I have a Rohloff hub with rim brakes (HS33, for the sake), with vertical dropouts, but I also have a single speed bike with horizontal dropouts.

I would suggest a procedure much like Daniel Hicks suggested, but in a different order, as to assure everything is fine. So, you should follow these steps:


Make sure the wheel is true AND symmetric. You can do it by finding out a bike with vertical dropouts, removing the tire from the wheel, and fitting the wheel with this second bike upside down. Measure the distance from rim wall to one of the sides of the frame. Then, you mount it in reverse (with sprocket pointing to the wrong side) to make sure that distance is the same. If not, the wheel is dished (should never happen with Rohloff), and you should use a spokes wrench to bring the rim to "center". Most probably, a bike shop is the recommended way to do it, but only if you think your bike shop's mechanic knows better than you (sometimes, they do not).
Assuming the wheel is symmetric, you should mount it in the frame WITH BRAKES REMOVED, in order to make the wheel centered relative to the frame. Since your dropouts are Rohloff OEM, you should: 


Replace your missing bolt in the first place (always use a washer!!); 
Loosen these bolts so that the dropouts slide freely; 
Tighten the wheel's quick release while the axle is fully inserted into the dropouts ON BOTH SIDES; 
With the chain engaged around the sprocket, slide the chain-side dropout fully backwards so the chain is properly tensioned, and tighten the two dropout bolts; 
Move the rim sideways, near the brake posts, so that there is the same distance between rim wall and frame tubes on both sides; 
Tighten the remaining two dropout bolts, on the other side;

Only then, install the brakes so that there is the same distance (the manual recommends 2&nbsp;mm) between brake pad and rim. Maguras are a bit tricky to adjust, so again, if you're in doubt, ask a mechanic, but remember that some mechanics (depending on the place you live) don't "capture" the subtle essence of correct setup of Maguras.


The steps described seem complicated, but they are necessary to guarantee, in this order, that your wheel is true, centered on the frame, the chain "tension" is correct, and the brakes are setup properly.

Your bike setup is fairly complicated because of Rohloff OEM dropouts, but mostly, I think, because you use rim brakes with moving dropouts. It means that, for every chain's lifetime, each time you have to compensate for chain slack, you have to mess with four small-sized Allen bolts, AND hydraulic rim brake setup. The brake has to be adjusted in this case because, if the dropouts move back, the rim moves back and the previous brake setup is not valid anymore.

Hope all this helps, and good luck! And PLEASE, replace that missing bolt!!!