Problem communicating with the fingerprint sensor TTL (GT-511C1R)

After I made sure the sensor was plugged in correctly and I tried to communicate with the sensor using an arduino uno and the arduino library found at GitHub - mdcanham/Fingerprint_Scanner-TTL-GT-511C1R: Repository for example code for SEN-11792 at SparkFun.com.
But the sensor just won't communicate with the the arduino. I was able to make the blink example work but only after uploading the code to arduino, unplugging the arduino from my pc and then plug it back in. But as soon as I open the serial monitor, it stops working and all the other examples won't work. I found that I wasn't the only one affected by this issue, but I haven't found someone who was able to find a solution. All the forum posts regarding this issue have been inactive for at least a year so I'm starting a new topic hoping I can get some help.

Thanks!

keep away from pin 0/1 when using the serial monitor.
Fingerprint_sensor uses softwareserial to other ping !?