This user have the same identical id as shell user.
But you must login with a '@' and with domain associated (p.ex.: user@domain.com)
If you still can not login, check if this user exists in '/etc/pure-ftpd/pureftpd.passwd' file, and if its home directory really exists, and if the file was created at the exact same datetime that the file '/etc/pure-ftpd/pureftpd.pdb'.