I have the same problem, search for hours and hours, but do not find any clue..
It was working fine months ago and touched nothing since, excepted the updates, but now that I need to seriously use it, it's not working anymore..
Also try to rollback to wp 8.5.4 and previous version of the plugin. it's not working.
The secure attachment was alredy disabled for me, but activated or not, it's the same.
I checked and reduced the .htaccess to the minimum, not have any other 'security' plugins.
Whatever I try, only when I deactivate the plugin I'm able to attach files.
If you have a clue... Because I do not find at all.. Thanks