Calculating currents with a current source in series
There is no law forbidding current sources connected in series to other elements. 

I5 must be generated by a current source to make the circuit work. Without a current source I5 is different from 5.0 A. 

If I5 is replaced by a current source with I5=5.0 A, you get the following voltages:
U1=U2=18V, U3=U4=(5*36/7)V=25.71V
Which means, your imaginary current source I5 must provide a voltage of -31,71V. This is rather uncommon for real current sources, but not impossible to build.

Otherwise the 5.0A cannot be explained.

(all voltages named after current indices, voltage counting directions according to current directions)