This bug is being tracked in Github as issue #71. Investigation/troubleshooting is underway; an update will be provided in this blog when a fix is available.
For now, try the following workaround:
- Open Clippings Manager
- Locate the clipping you want to paste into Gmail, and select the desired clipping in the Clippings Manager tree list
- Select the text in the clipping that you wanted to paste into Gmail, then drag and drop it into the message editor in Gmail