It seems there is a bug with Wordfence's treatment of htaccess.
After having activated the plugin's falcon cache, I changed the htaccess to add default character set in the HTTP Response.
It worked, but a few hours later I was informed that the text did not look good on my site with strange characters.
I assume Wordfences' caching mechanism somehow recreated some htaccess without taking the changes into account, which would be a bug.
For now I deactivated falcon, cleared cache, changed htaccess as I wanted and reactivated falcon.
I cross my fingers and hope it will work, otherwise I'll continue this thread.