Hi Barry,
Having the same issue with some of our Wordpress websites.
We found that only certain websites had this issue and these websites all had WP Supercache Installed, although it was deactivated, we noticed that the code it creates was still present in the .htaccess file.
It seems this code conflicted with the Wordfence plugin when we chose the Falcon option and caused this error. So I am wondering if you have a similar issue. Do you have any other cache plugin that is putting data in your htaccess file, is your issue fixed when you remove the code created by the other plugin?