I2C external MAG Sensor blocks all other I2C Sensors - MAG broken oder I2C Port?

Hello guys.
My name is Sandro and i build Multicopters.

Currently i have a huge Problem with an external MAG I2C Sensor.

So i have a Arduino Mega based Flightcontrol - http://www.rctimer.com/index.php?gOo=goods_details.dwt&goodsid=765&productname=

Sensors:
MPU6050 6-axis gyro/accel with Motion Processing Unit
HMC5883L 3-axis digital magnetometer
MS5611-01BA03 - Barometer

Also this Board has an external I2C Port (Gnd, 5V, SDA, SCL)

Since i experienced problems with the internal Mag (High Voltage, magnetism ...) i wanted to remove the internal mag and put the exact same I2C Mag Sensor (HMC5883L) on the external I2C Port.
Some other owners did it before with success.

So i bought the excact same MAG Sensor on BoB: www.ebay.de/itm/HMC5883L-3-Axis-Compass-Magnetometer-Breakout-Board-/280863037681
I soldered all 4 wires to the right position, i am very sure about that i checked it 10 times.

I fully removed the internal MAG from my board using a scalpel and after that the board worked fine.
I tested the board, i went out for a flight, everything was great - just without MAG.
Then i connected the external MAG to the I2C port and suddenly all I2C sensors stopped working.

Since i know that those kind of issues need as many informations as possible i flashed an I2C Scanner to my Arduino Board.
When i removed the external Mag - The I2C Scanner found 2 I2C Sensors: 0x77 and 0x68 (So MPU and Baro)

But when i connect the external Mag to the I2C Port and start the I2C Scanner - there is no output at all so the whole I2C stops working.

So my Question is: Is the Mag BoB broken or my external I2C Port? Or did i forget something?
Is it even possible that a broken I2C BoB forces the whole I2C to collapse?

I hope you can help me.

Greetings from Germany
Sandro

Solved using another MAG - The bob is broken or creates an electrical short