I have at my hands several devices communicating to a Raspberry Pi over serial links. One of them connects to Pi's own UART, and thus is usually found at ttyS0. Another one connects over USB via an FT230X chip, and thus usually gets ttyUSB0. No idea what chip the 3rd one has, but it pretends to be CDC, and so usually lies at ttyACM0.
This setup worked perfectly with the slightly outdated installation of Raspbian 10 Buster that the Pi was running up to now. However, we're planning to move this particular system to Ubuntu 20.04, so I gave it a trial run. Much to my dismay, when running Ubuntu, I was unable to write to either ttyS0 or any ttyUSB* ports. To elaborate - I can open them from any program just fine (for example, minicom), and if the device at the other end is sending something, the Pi receives without problem, but trying to send anything from the Pi has no result - not even blinking the TX light on a spare FTDI USB-UART dongle I hooked up for tests. CDC-ACM devices still work fine. I should note that for the purposes of testing, I was logged in as root - and I checked the permissions on device files, looked like I should've had both read and write.

I'd be very thankful if someone could point me in the general direction of a solution. Failing that, does anyone happen to know a USB-UART that can do CDC? I have several spare CH340s and some oddball Prolific device, but these all get picked up as ttyUSB.

On the highest level as I understand how it works.

Prolific and FTDI devices are recognized and use the USB-serial driver.

CDC ACM devices are recognized and use the cdc-acm driver.

From the users stand point they both behave the same but are different hardware and its possible there could be something wrong with the usb-serial driver in Ubuntu's arm version.

With raspian/Pi OS I have tried both FTDI and Prolific devices and they work fine but I never have tried running something different yet.

There are some notes on how to switch the drivers to generic but without flow control would not be final solution.
https://www.kernel.org/doc/html/late...sb-serial.html

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Excuse the slight necromancy - just figured I should leave a response to michaelk and the poor soul reading this some 7 years later on the night before a deadline. Long story short, it was decided that getting stuff running on Ubuntu has relatively low priority, so there has been no time to try out the generic drivers yet - will update when it finally happens.