Finder Connection to NAS unstable

[Szyfrows]

Hi, Sinds a few weeks Finder is not stable anymore (I'm using Mavericks and a Synology DS213J NAS). For instance, if I try to duplicate a file from the context menu, Finder reverts to a standard folder on my computer. The same happens when I open a file: the file opens OK, but Finder reverts to the standard folder.

I already tried the following:

• Deleting finder.plist – didn't make a difference.
• Using smb, afp or cifs – no difference
  
• I tried logging on to my iMac as another user (no startup items) – same results.

Under Yosemity inside a VMWare Virtual machine, Finder works fine. Of course, upgrading to Yosemity might solve the problem, but I'd rather not upgrade at this point.

Thanks for any ideas!

Configuration:
OS X 10.9.5 Mavericks
Synology DS213J
DSM 5.2-5644