Summarizing the discussion, it looks like this bug can be fixed by simply replacing the line
#NAME_REGEX="^[a-z][-a-z0-9_]*\$" in adduser.conf by #NAME_REGEX="^[a-z][-a-z0-9_]*$"
Best, tueftler11
I can confirm this bug. I just came across this on Ubuntu 20.04. I
prefer the proposal by Laurent Martelli.
