Problem transferring files between itunes libraries: error -50

I recently replaced the external HDD that is connected to my (headless) Mac Mini and which I use as media center, with a WD 6TB Mybook (USB 3.0). The external HDD is used to store my Itunes Library.

The Mac Mini runs OSX Mavericks 10.9.5 and Itunes 11.4 (I did not upgrade to Itunes 12 because I do not want to lose the option to easily drag and drop between libraries in the left column..).

With my previous external WD HDD I never had file transfer issues, but I am now frequently having issues when I try to transfer files from Itunes (11.4.0.18) on my PC to Itunes on the Mac Mini and also when I try to transfer files via the file transfer option in my VNC Client from the PC to the Mac. I can see/read the content of both libraries, but file transfer is not possible. The error message I get is 'An unexpected error occurred (error code -50).'

However, if this happens and when I restart the Mac Mini, I can tranfer files again. Unfortunately most of the times the problem is back the next day (the Mac Mini is set to go into sleep mode during night hours).

Both the Mac Mini and my PC have fixed IP-addresses in my home network.

Any thoughts? A problem with rights on the external HDD perhaps, and any advice on how to check and fix this?

The Mac has an admin account, but the account that is set to automatically launch and that is always running Itunes is a standard user account. I had the same account setup when I had the previous HDD and this never led to problems.

Thank you.