I finally figured this out. Turns out the FreeBSD side of things worked from the start, and it was the hardware that was broken. A replacement adapter works like a charm.
Oh and yes, Oliver, you're perfectly right, /dev/cuaU0 is indeed created by ucom. I was confused by the device having the creation date of when the system was booted, even though it was dynamically created a couple of days later. Thanks for all your help! Cheers Benjamin
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