Weird problem with string and library

I'm trying to build a radio, with a 5110 display (SPI), RTC and Si74703 (I2C) but it does not seem to be working.

#include "LCD_5110.h"
#include <Si4703_Breakout.h>
#include <Wire.h>
#include "RTClib.h"

/////////////////////// RADIO
int resetPin = P1_4 ;
int SDIO = P1_7;
int SCLK = P1_6;

Si4703_Breakout radio(resetPin, SDIO, SCLK);
int channel;
int volume;
char rdsBuffer[10];

////////////////////// CLOCK
RTC_DS1307 RTC;
DateTime now = DateTime(__DATE__, "01:01:01") ;

////////////////////// LCD
LCD_5110 lcd_screen;
boolean    backlight = false;

void setup()
{
 lcd_screen.begin();
 
 lcd_screen.setBacklight(backlight);
 lcd_screen.text(0, 0, "Initializing");
 
 Serial.begin(9600);
 
 Wire.begin() ;
 RTC.begin();
 
/* radio.powerOn();
 radio.setVolume(3);
 */

 //if(!RTC.isrunning())
 {
//  RTC.adjust(DateTime(__DATE__, __TIME__));
   RTC.adjust(DateTime(__DATE__, "01:01:01"));
 }

 lcd_screen.clear();
 lcd_screen.text(0, 0, "Done");
 
 displayTime() ;
 lcd_screen.text(0, 5, "Kikou");
}

void loop()
{
 displayTime() ;

 delay(1000);
}

void displayTime()
{
 lcd_screen.text(0, 0, "Test");

 now = RTC.now() ;

 lcd_screen.setFont(1);
 String time_str = leadZero(now.hour()) ;
 time_str.concat(":") ;
 time_str.concat(leadZero((int)now.minute())) ;
 
 lcd_screen.text(0, 0, time_str) ;
 String min_str = "" ;
 min_str += ":" ;
 min_str += leadZero(now.second()) ;
 
 lcd_screen.setFont(0) ;
 lcd_screen.text(10, 1, min_str) ;
 Serial.println(min_str) ;
}

String leadZero(int val)
{
 String str = "" ;
 if(val<10)
 {
  str = "0" ;
 }
 str.concat(val) ;
 return str ;
}

when it display the time I got

01:0 :0

on the LCD or on Serial.print and when the number of second id >= 10

01:0 :10

if I comment out

Si4703_Breakout radio(resetPin, SDIO, SCLK);

I got the right display

01:01:01 .... 01:01:02 ... 01:01:15 etc 

The RTC device and the radio are on I2C both, but the constructor of the radio object does nothing...

Before this version of code I got all sort of problem with the strings manipulation of the time.

Looks like an memory problem between String and one or more of the library to me....