Linux serial ports only work at standard speeds by default. MIDI runs at 31250 baud, which is not a standard speed. However there are tricks to get custom speeds, but documentation is quite fuzzy. This is a simple recipe that worked for me with an FT232 USB-Serial board.
Check the baud base of the device:
$ setserial -g -a /dev/ttyUSB0
/dev/ttyUSB0, Line 0, UART: unknown, Port: 0x0000, IRQ: 0
Baud_base: 24000000, close_delay: 0, divisor: 0
Flags: spd_normal low_latency
As you can see the baud base is 24000000 here.
Next calculate the divisor by dividing the baud_base you see here by the speed you want.
In my case 24000000/31250=768.
Apply the new setting:
$ setserial -v /dev/ttyUSB0 spd_cust divisor 768
Next start your serial application, you might want to make sure it is already set to the correct speed before you do the above changes else it might destroy your settings. The correct speed is 38400 baud, which is now aliased to 31250.
With minicom just use the menu (CTRL-A Z) to change the settings.
Screen can be used like this:
screen /dev/ttyUSB0 38400
Exit screen by pressing CTRL-A CTRL-\