Occasionally drag'n'drop failures —where it just stops working— I experienced already with Yosemite, so it may be a VERY old bug. :) Usually you would see "CoreDragError" codes popping up in the system.log

Rather than rebooting, I just kill "pboard" in Activity Monitor, and afterwards restart a few GUI programs that were directly affected (including Finder and Dock.)

My wild theory is that Firefox is the culprit. At least the issues happened more frequently once I had to switch to Firefox for proper content blocking extensions. Anyway… I have no idea how to prove that, so it's just a guess, not a blame.