In /etc/login.defs, ensure SHA_CRYPT_MIN_ROUNDS and
SHA_CRYPT_MAX_ROUNDS has the minimum value of 5000.
For example:
SHA_CRYPT_MIN_ROUNDS 5000
SHA_CRYPT_MAX_ROUNDS 5000
Notice that if neither are set, they already have the default value of 5000.
If either is set, they must have the minimum value of 5000.