Voltage divider (phasor domain)
The potential difference \$V_u\$ is the difference between the voltage drops across the two resistors in the two legs. Hench they have applied 2 voltage dividers to find the potentials across each of the resistors.

$$V_{R2}=V_e\times\frac{R}{R+\frac{1}{jwC}} = \text{potential across the resistor in the second leg} $$

$$V_{R1} = V_e \times \frac{R}{R+R}=\text{Potential across the resistor in the first leg}$$

\$V_u\$ is simply the difference between the potentials above.



In the method you have used you are mistaking what \$V_u\$ stands for and thats where you are going wrong.