I'm experiencing this problem right now with the Minimal theme. I can paste the text anywhere else within my browser or my computer, just not into the product copy box. If multiple themes are seeing this problem, it sounds like it might be more wide spread.
Hey all - I did find a workaround in this thread that at least works on Minimal - if you have access to the HTML, paste that in on the code side. Worked yesterday and again this morning with no issues.