You can try reset the Lightroom Preferences to default and see if that helps.
Go to : /Users/[user name]/Library/Preferences/com.adobe.Lightroom5.plist
Rename the file "com.adobe.Lightroom5.plist" to "com.adobe.Lightroom5.plist.old"
Open Lightroom with same catalog file and try Copy & Paste feature.
Let us know if that helps.
I'm having the same issue again on both of my Macs now. Suggestions? It's a pain when I have as many as 500 images to correct and I can only do them in small batches.