I am having the same problem. I installed a system upgrade to my Mac in advance of the El Capitan release about a week ago and it seemed to break my DNS-320L. With AFP enabled, the NAS used to display as "Volume_1" under "Devices" in the Finder sidebar. While performance was not as fast as in Windows, it was at least functional. Now AFP doesn't seem to work at all and the NAS is super slooooooowwwwwww. The alias under "Devices" stopped working entirely. It would say "Volume_1" is not found. I can still access the NAS under "Shared" in the Finder sidebar if I look in "All" network places. However, It is barely functional. Moving data across takes hours and simply browsing the file structure requires 10 to 15 minutes to open a folder. I upgraded to the latest firmware (1.04) and upgraded the OS to El Capitan yesterday, to no avail. The NAS continues to be worthless. If I can't get this fixed soon, I will have to buy new hardware and junk my DNS-320L. I'll need to install Windows in Bootcamp just to copy off my data. It'll be a major chore. I've logged into the DNS-320L and restored it to factory settings, also to no avail. I've turned AFP and Time Machine on and off under System Services. I have network shares created and users. Nothing works.
I was never real satisfied with the DNS-320L using Mac OS X. It works much better with Windows. I wouldn't recommend any D-Link products for Mac users. They are made for Windows. Stay away.