While trying to use the screen the second time (the first time I just close the terminal without exiting the screen with ctrl + a + k) I got this error:

$ screen /dev/ttyACM0 115200
[screen is terminating]

After some searching I found the solution:

$ sudo lsof /dev/ttyACM0
lsof: WARNING: can't stat() fuse.gvfsd-fuse file system /run/user/1000/gvfs
Output information may be incomplete.
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
screen 62004 alan 5u CHR 166,0 0t0 735 /dev/ttyACM0
$ kill -9 62004

Source: https://unix.stackexchange.com/questions/93892/why-do-i-get-screen-is-terminating-without-root