> adduser in Debian doesn't seem to be very responsive to our bug reports, so we may need to fix it ourselves...
Whether this will be fixed by yourselves or by Debian, the problem need to be understood first.
Is there a way to reproduce the issue? Does anybody have an idea if this lock file was really left by useradd?
The lock files are actually created by useradd, but they should also be removed by useradd.
Note that after a system crash anything could happen. useradd might have been killed between the creation of the lock file and its removal. In that case, the lock file is a feature which indicate to the user that the system files should be checked.
The only bug I can see in this bug report is that the actual message "cannot lock /etc/passwd" is not displayed to the end user and the usermod failure seems to be ignored silently.