this expression redefines a previously defined functions

Thats a tricky one. The problem is that the symbolics will trace back all definitions how a variable is derived and will evaluate this calclulations symbolically. In your case it will choke at the definition of Xp. Xp is derived by using lsolve and lsolve cannot be evaluated symbolically.

Workaround is to cut the connection between Xp and lsolve by using a redefinition with an inline assignment:

Not all works as expected and you see the symbolic function definition

P.S.: I see you found the correct value for x.N in your routine X_Gauss yourself. Better replace it by a more general expression using the argument U