The problem may be an incorrect line in the passwd file anywhere above the line
on which the user is listed. In this case, the passwd file entry 10 lines above
was missing a colon. Replacing that colon fixed the problem.

To determine WHERE in the passwd file the problem is, make copies of both the
passwd and shadow files, then enter

pwconv

Then, when you look at the passwd file, you will see that the line with the
problem
and all lines below that point will be missing. Now, of course, you'll have to
copy back the original passwd and shadow files, but you'll know what line the
syntax error is on.