Error message when copying files over to Mac Server

I keep receiving the following error message when copying files, from my iMac and my Macbook, over to my Mac Mini Server.

The Finder can’t complete the operation because some data in “filename.pdf” can’t be read or written. (Error code -36)

But regardless of the error message, single files will still copy over. Multiple files however, will copy the first file, then kick the error message and stop.

Really a pain when I have a folder with 100's of images and I have to copy each image over individually!

However, when I copy files from a PC Desktop or Laptop to the Mac Server, no error message occurs.

I've been dealing with this issue for months and have not found a solution. I get the same message no matter what type of file it is.....PDF, JPG, DOC, Ai, etc.

Can anybody help??