OpenCM9.04 with Arduino IDE "eneter bootloader"

Hi there,
I'm trying to program my OpenCM9.04 board with Arduino IDE, but unsuccessfully.

I followed these instructions ( Arduino IDE) on how to set up the Arduino IDE.

Despite all that, I still can't upload a simple b_Blink_LED example from OpenCM9.04 examples.

When I try to upload the sketch to my board, I get stuck on infinite "Uploading" status and a message "Enter bootloader" occurs.

I checked, that I have the OpenCM bootloader selected as "Upload method".

I believe it has something to do with the fact, that I'm trying this on a virtual machine (Virtual Ubuntu 18.04 via Virtualbox), since when I tried the same thing on my host system(Win 10), it worked completely fine.

But the problem shouldn't lie with Arduino IDE itself, since I can normally upload code to my Arduino MEGA without problems.

Any ideas, on what might be causing this error?

I'll be happy for any ideas or suggestions.
Thanks in advance!