Using c_str() on String for IPAddress with NTPClient provides wrong values
The String object created as return from IPAddress.toString() as parameter to constructor of NtpClient is temporary. It contains the char array returned by c_str(). NTPClient doesn't copy the string, only stores a reference to it. And the referenced string (char array) doesn't exist at the time the NTPClient wants to use it.

class TestClass {
public:
  TestClass(const char* _ip) : ip(_ip) { }

  void test() {
    Serial.println("--");
    Serial.println(ip);
    Serial.println("--");
  }

private:
  const char* ip;
};


//String s("abc");
//TestClass Test(s.c_str()); &lt;- this works

TestClass Test(String("abc").c_str()); // &lt;- this can't work

void setup() {
  Serial.begin(9600);
  Test.test();
}