EDIT: but a c++ program seemed to be able to access the port based on the voltages, but only *sometimes*.

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I read through dozens of threads here and elsewhere, and I'm sure this is a common problem...but I can't find relevant threads?

Basically I can't even use minicom to write to the serial port. An oscilloscope shows no signal being outputted when typing on minicom. I tried ttyS0-ttyS4 just in case, though my computer only has one physical port. I have a loopback adaptor (TX/RX connected) so what I type should be returned.

Minicom is set up for /dev/ttyS0 for 57600 8N2.
The port is there, since "dmesg | grep tty" returns:

serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A

And "ls /dev/ttyS*" returns:
/dev/ttyS0 /dev/ttyS1 /dev/ttyS2 /dev/ttyS3

Why is there no output on minicom when I type?

I'm new to linux, and am on Gnome/RedHat...

Try dropping it down to 9600 baud ... from memory it's 'minicom -s' to set defaults