This is related to the OSX/Pirrit thread from here. After two days of debugging and visiting Apple stores, I eventually erased all data on the disk and migrated my files later with Migration Assistant. It's fixed now! However...

I can't use my old username! If I rename the home folder to that one, the pervasive error shows up again. It's really annoying.

I'm not sure if this is some kind of malware, as I ran the command indicated by @klanomath:

dscl . -list /Users UniqueID | grep 401

And nothing was printed in the console, thus this shouldn't be OSX/Pirrit. What might it be then?

Running High Sierra 10.13.5 on a MacBook Pro Late 2016.

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