“Can only flip STRING and INTEGER” Error

Hi,

Few days ago Permalink manager stopped redirecting links, instead i was getting 404 for the redirected links. and i’m getting the following error:

PHP Warning: array_flip(): Can only flip STRING and INTEGER values! in wp-content/plugins/permalink-manager-pro/includes/core/permalink-manager-core-functions.php on line 171′, referer: https://website.com/wp-admin/edit.php?post_type=product

And this is only one error. there are many in other areas too.

The only thing that was changed is newly installing W3 Total Cache. however i tried to disable that plugin, but the same issue kept occurring.