Backgorund:
Drag-drop suddenly stops working.
Temporary solutions:
4-5 reboots
Removing com.apple.finder.plist file
Have not tried:
Reinstall Yosemite
What I found out:
I found out that it's not only about drag/drop - it's about the clipboard. As far as copy/paste went, "nothing" work anymore. Not drag and drop, not cmd+c, not Edit > Copy. I started experience the issue just yesterday I think - it came out of the blue.
However, a week ago I created an Automator scripts that review folder X for extension Y and then moves the file of said extension into a separate folder. Essentially, it's a cut and paste job that Automator does all the time.
Soluton:
Removed the Automator workflow i created from /users/username/library/workflows. Rebooted computer. Drag/drop and copy/paste has been working for over an hour.