According to my research the problem is not DSM but MAC OS. Here is what I suggest.
Open Finder and go to where the directory you want to copy to your NAS is. Then press "Command + Shit + ." (dot). That will show all hidden files within MAC OS finder. Now browse your directory and sub-directories. If you see any .AppleDouble, .AppleSingle files or @eaDir directories they will need to be deleted. They normally are the source of the error 50 you are seeing. If you see many of those files and you have loads of sub-directories then you can do this in one go through Terminal, otherwise you can delete them manually. If you use Terminal then you can issue the following commands:
Change directory to the directory want to copy:
cd [directory-path]
Now check in which sub-directories the files are.
find . \( -name ".AppleDouble" -o -name ".AppleSingle" -o -name "@eaDir" \)
If those files are present you will see a list of paths.
Now you can delete the files:
find . \( -name ".AppleDouble" -o -name ".AppleSingle" -o -name "@eaDir" \) -exec rm -rf {} \;
Deletion is permanent so be very careful you are on the root of the directory you want to work with otherwise the search will expand to wherever you are.
Alternatively you could explicit the full path of your directory:
find \[directory-path] \( -name ".AppleDouble" -o -name ".AppleSingle" -o -name "@eaDir" \)
find \[directory-path] \( -name ".AppleDouble" -o -name ".AppleSingle" -o -name "@eaDir" \) -exec rm -rf {} \;
Obviously, replace [directory-path] with the actual path of the directory.
To hide hidden files simply press "Command + Shift + ." again.
If you are unsure of something it is better to ask first.