Accuracy of MSI (Timing problem with LMIC)

Hey there. 

I am experimenting with LoRaWAN (the things network) and the STM32L053C8 discovery board.

I set up my sysclock to MSI 2 MHz and the systick interrupt to count up 10000 times a second (100us ticks). 

This is required by the lmic library to provide accurate timing for 2 receive windows.

The problem is, that both of the receive windows time out every time. So the question is if it's possible that the clock is that inaccurate or maybe i did some wrong in the configuration.

Would be very nice if someone could have a look at my code.

Code is attached as eclipse project.

Thanks in advance.

Kind regards, Rob