All buttons on Campaign Creation page generate a 403 Forbidden message

phpList Version 3.3.3 , php version 5.6.37, shared host environment, LAMP environment

I’m trying to create a new campaign. After typing the message, pressing any of “Save as Draft”, “Save and Continue Editing”, “Send (a test messge)” or any of the tabs at the top, returns a 403 Forbidden message with "You don’t have permission to access /phplist/admin/ on this server. "

I was getting this message just prior to the upgrade to 3.3.3., and tried the upgrade to see if that would fix the issue… it didn’t…

We’ve been using phpList for a long time now, and this is a new issue in the last 48 hours for us. We’ve not made any changes to config on phpList during that time period.

Any help would be greatly appreciated