Have you downloaded the ‘User Access Manager’? I was having this same problem and it started once I downloaded that. I had tried everything. In the settings section of the ‘UAM’ pluggin I turned off the ‘lock files’ setting all together and I still got a pop up window asking for a login for ‘Authentication Required’.
So, I came here looking for answers and came across your post. I decided to go back to the ‘lock files’ section and look at the .htaccess thing. I turned the ‘lock files’ to on and set it to not lock files that were image related. That seemed to fix it for me.
I don’t know if this is what you’re looking for, but hopefully it helps… if you haven’t already figured something out.