Serial communication on Linux works using mono, but not inside of Unity

Hi,

I am having some trouble with serial communication on Linux. I am trying to send and receive information between an arduino and Unity. I suspect that it might be a Linux permission thing, but it might also be some Unity setting that I have missed. I already added the Linux user to the dialout group as per instructions on the Arduino website, and I made sure to select .NET 4.x in the Unity project settings. The serial communication is actually working when I compile and run the following C# using mono, outside of Unity:

 using System.IO.Ports;
 sp = new SerialPort("/dev/ttyACM0", 9600);
 sp.Open();

However, the same code in Unity at run-time tells me:

 IOException: No such file or directory

The frustrating thing is that this is actually working on another Linux machine and I am having trouble understanding what the difference could be that is causing it not to work on the other.

Here are some differences on both systems:

The working Linux is Lubuntu 19 with the Unity installation inside of /home, which is on the same partition as root. The Unity version is 2019.2.

The non-working system is Linux Mint 19.3 with the Unity installation also in /home, but this is a different partition from root. The Unity version is 2019.3.

The permissions look slightly different too:

 crw-rw----+ 1 root dialout 166, 0 mei  1 05:08 /dev/ttyACM0    --> Lubuntu
 crw-rw---- 1 root dialout 166, 0 May  1 15:03 /dev/ttyACM0     --> Mint

Also:

 getfacl /dev/ttyACM0

gives me the following on Lubuntu (working):

 # file: dev/ttyACM0
 # owner: root
 # group: dialout
 user::rw-
 user:myname:rw-
 group::rw-
 mask::rw-
 other::---

and the following on Linux Mint (not working):

 # file: dev/ttyACM0
 # owner: root
 # group: dialout
 user::rw-
 group::rw-
 other::---

Does anyone have any insight into why in one scenario Unity can't access /dev/ttyACM0, while in the other one it can? Or any idea's how I might find out? (Also, I wanted to test this out with the same 2019.2 Unity version, but for some reason my Unity Hub does not want to load any archived installation).