CiviCRM 4.6.8 compatible with Wordpress 4.3?
This doesn't look like a Wordpress compatibility error.  It is, as you guessed, related to deduplication.

It sounds like you may not have a rule designated as your "Supervised" dedupe rule - or there's a problem with the rule you have designated as your "Supervised" rule.  Check to make sure you have a rule, and if you do, try changing it to the default Supervised dedupe rule.  That should clear your problem.  Then you can troubleshoot the dedupe rule itself.