When importing memberships, I get "Error: Invalid value for field(s) : Start Date; End date"
Are you using WPSec or some other security related wordpress plugins? Check:

https://wordpress.org/support/topic/plugin-better-wp-security-client-denied-by-server-configuration

Seems like:

a. CiviCRM is trying to write something to the log file

b. The security plugin traps that and redirects you back to step 1

try disabling the plugins and see if that helps