We have an on premise SharePoint 2013 installation with two authentication methods - Forms and Windows (NTLM). Using a Windows login everything is fine but when using a forms based we get the following error message when trying to open Nintex forms designer from a list "An error occured during an Ajax call". After clicking ok all menues etc. are not showing and we are not able to use Nintex forms designer. But Nintex workflows doesn't give us any problems. Any suggestions?