Problems with Mega256, I2C and Braccio Shield V4 (Arduino shield for robot arm)

I have a Braccio shield V4 connected to a Mega2560 rev3 the Mega is communicating to a device using I2C.
the communication to the I2C works fine until I plug in the Braccio shield, then the Mega hangs up when I execute the command: Wire.endTransmission(false);

If I unplug the Braccio shield, the I2C works fine

Any Ideas?

here is the code segment:

Serial.println("after begin transmission");
binaryFloat ms;
Serial.println("after write command");
ms.FloatingPoint = f1;
ms.FloatingPoint = f2;
Serial.println("after writing 2FP bytes");
Serial.println("after ending transmission");

Here is the Serial output:
after begin transmission
after write command
after writing 2FP bytes

the Mega completely hangs up at this point

Read the how to use this forum-please read sticky to see how to properly post code and some advice on how to ask an effective question. Remove useless white space and format the code with the IDE autoformat tool (crtl-t or Tools, Auto Format) before posting code. Post the entire program if possible.

Are you connecting the I2C device to Mega pins 20 and 21 or the I2C connector on the shield?

Yes, I am using the pins 20,21 for the I2C communication.
(sorry about the formatting of the code)

Guide for Braccio shield:
I found the schematic for version 1: Tinkerkit Braccio robot | Arduino Official Store (under tab "DOCUMENTATION").

Th SDA and SCL next to the AREF pin are connected to pin 20 and 21 on the Mega board.
They go to a I2C level shifter chip on the shield. There are 10k pullup resistors on both sides of the that level shifter.
The Arduino Mega is the only board that has onboard 10k pullup resistors for SDA and SCL.
So there is at least a strong 5k pullup to 5V on SDA and SCL, eagerly waiting to destroy a 3.3V sensor via the SDA and SCL pins.

Are you using the newest Arduino IDE 1.8.13 ? or a very old version ?

Can you measure the 5V pin of the Mega board ? It should be above 4.5V because the ATmega2560 does not like low voltages.

Did you set the power switch to 5V on the shield ?

What is the other device that you have connected to the I2C bus ?
You may not connect a 3.3V sensor to the Mega board without I2C level shifter. Do you use a cable ? Do you use extra pullup resistors ? are there pullup resistors on the modules of that I2C device ? Can you give a link to that module ? (a link to where you bought it please).

The Wire.endTransmission() gets stuck when SDA or SCL is shortcut to GND or SCL is shortcut to SDA. Can you check with a magnifier if the SDA and SCL pins on the shield are shortcut ? with a copper trace or a solder blob. Or test it with a multimeter.

Please keep the discussion here and not over there:

After reading blh64's post, you need indeed to release the I2C bus with the last as: Wire.endTransmission(); (without parameter or with true as parameter).