No permission to access my usb device, which is in /dev/ttyUSB0, even though I am in dialout

It's expected that you cannot change this file's permissions as a regular user because you're not this device's owner. Nothing out of the ordinary here.

With regards to symlinking the /dev/ttyUSB0 as /dev/ttyS0 - I think it's a bad idea. Unless you have a source of the program trying to use the port, you don't really know how the developer checks for the device node of the serial port. Maybe it checks the device is a character device (symlink doesn't satisfy this condition) and then fails?

I'd always prefer using the program command line flag to specify a port over making workaround such as symlinking.

ArSerialConnection::open: Could not open serial port '/dev/ttyUSB0' | ErrorFromOSNum: 2 ErrorFromOSString: No such file or directory
Could not connect, because open on the device connection failed.

What's the output of stat /dev/ttyUSB0 when you run this command?

Also - after adding your regular user account to the dialout group, did you log out and then back in?