C++ no match for operators

Question

Ive got a problem that I cant use operator overloads even though I declared them. I've got a class CKomplex for complex numbers:

typedef double number;
class CKomplex
{
private: // member variables
number mReal; 
number mImag;

public:
CKomplex(number real = 0);
CKomplex(number real, number imag);
CKomplex(number radius, number phi, bool isRad);

// Operator
void operator+=(CKomplex &rhs);
void operator-=(CKomplex &rhs);
void operator*=(CKomplex &rhs);
void operator/=(CKomplex &rhs);
void operator=(CKomplex &rhs);    // = 

friend CKomplex operator+(CKomplex  &lhs, CKomplex  &rhs); // +
friend CKomplex operator-(CKomplex  &lhs, CKomplex  &rhs);
friend CKomplex operator*(CKomplex  &lhs, CKomplex  &rhs);
friend CKomplex operator/(CKomplex  &lhs, CKomplex  &rhs);

// Get & Setter
number GetReal() { return mReal; }
void SetReal(number real) { mReal = real; }

number GetImag() { return mImag; }
void SetImag(number imag) { mImag = imag; }
}



// Here is the implementation
void CKomplex::operator=(CKomplex &rhs) {  // no friend
    mReal = rhs.GetReal();
    mImag = rhs.GetImag();
}

CKomplex operator+(CKomplex  &lhs, CKomplex  &rhs) { // friend
    number real = lhs.GetReal() + rhs.GetReal();
    number imag = lhs.GetImag() + rhs.GetImag();
    return CKomplex(real, imag);
}

But the code wont compile when I write this in my main() function:

CKomplex z1 = CKomplex(5,3); // works
CKomplex z2 = CKomplex(8,5); // works
CKomplex z3 = z1 + z3;       // works; but why?
z3 = z1;                     // works
z3 = z1 + z2;                // doesnt work line 30
z3 = (z1 + z2);              // doesnt work also

With the error:

C:\Users\User\Documents\soExample\main.cpp:30: Fehler: no match for 'operator=' (operand types are 'CKomplex' and 'CKomplex')
     z3 = z1  + z2;
        ^

(Arrow at the =)
Its my first C++ program, Im using QT 5.6.0 as console application on Windows 7.

If you need further information feel free to ask, I also can give you the whole source code.
Why does line 3 compile but not line 6 in my main()? Where is the difference for the operators?