After a lot of testing and debugging of the servo module, I discovered I made a horrible mistake in the module project. I forgot the I2C pull-up resistors. Fixing it was not easy, but was done. I soldered two 10k resistors over the SDA and SCL lines of the servo module and connected them to the 3v3 line. Another big problem I faced was the PCA9685 I2C addressing which was calculated in the wrong way by me. The logic level analyzer I bought at DX.com for 9USD helped a lot to find and fix the error.
It the spare time of two weeks to find and fix all the errors, but now the module has 16 working PWMs ready to control the servos.The prototype can be seen working in the video below.
Next step is to plug the module in the YAUVC backbone and start controlling it via WIFI, or let the flight control module take care of it.