File access confusion with afp and smb

My environment is all Mac. Years ago I read that afp was the best access method in an all-Mac environment and my ReadyNAS 516 was set up that way. I've since learned that it's preferable to use smb. So, I added smb as an access method. I generally use that.

The problem is that I occasionally run into a file that can only be accessed via afp. When I use smb I get this message:

"filename" can’t be opened right now because it’s being used by another task, such as moving or copying an item or emptying the Trash. Try again when the current task is complete.

First of all, nothing is using these few files. What's confusing is that, other files in that same directory can be accessed fine. AND another Mac on the network accesses all of the files still using afp.

This problem is on a new Mac using Sierra. I figure that this is a problem with the Mac and not the ReadyNAS because when I go into Terminal and run "ls -l filename" all the files in that directory show the same access rights.

Are there any Mac experts here who may have an idea how I can fix this problem on this Mac? In the past I would have tried "Repair Permissions" but since El Capitan it's no longer available.

Thanks for any ideas you might have.