so there is no current flowing from secondary through the fedback resistor like in my drawing in blue?
i thought, becaue of the high input impedance of the opamp,
the current schould flow in the direction which i have drawn...
When I say currents flow into and out of the inputs of the op-amp, I am talking about bias currents of a non-ideal op-amp. If we are talking about an idea op-amp then that does not happen.
For the current you have drawn, think about how the current makes a loop to return to the sources. It doesn't have a way to do that.
I think that current in the feedback resistors around the center resistor in the divider, not into the secondary itself. I know it's a little weird. It's because half your circuit is floating. Your diff-amp doesn't really care about anything except for the voltage across the center resistor. If that voltage is zero, your diff-amp is happy.
The only place where current from both the secondary and diff-amp flow is the center resistor in the divider. It's the only way the currents can form a loop. The secondary drives a current through the entire resistive divider (and in a non-ideal op-amp a tiny little bit of bias current in and out of the inputs of the diff-amp so that the diff-amp can actually have signal voltages to function), while the diff uses it's +/-5V to drive a counter-current through the center resistor so that the voltage across the center-resistor is zero. So basically the diff-amp tries to make the three-resistor divider look like a two-resistor divider to the secondary.
Replace the secondary with a voltage source. That might makes things clearer for you.