Cannot connect to Plesk via FTP: unknown configuration directive ‘IdentLookups’

Reason :

Atomicorp ProFTP service is not compiled with the mod_ident.c module.

The fix was released by Atomicorp on version psa-proftpd-1.3.7rc3-6.el6.art.x86_64.
Resolution

Log in to Plesk via SSH

Update the ProFTPD package to psa-proftpd-1.3.7rc3-6

Then run : yum update psa-proftpd