Superposition to find voltage U2
Voltage \$U_{A2} = (1+\dfrac{R_6}{R_1})\cdot U_{R_5}\$

And, \$U_{R_5}\$ (when U3 is shorted) is \$\dfrac{R_5}{R_4+R_5}\cdot U_{R_3}\$

And, \$U_{R_3}\$ (U3 is shorted) is \$\dfrac{R_3||(R_4 +R_5)}{R_3||(R_4 +R_5)+R_2}\cdot U_2  \$

Does this allow you to check your answer or is there still a discrepancy?