There is some text present in the rich text editor field. Either a previous post being edited, or a current post, not yet made, partly complete. etc....
Steps to recreate:
Highlight some or all of the text and copy it (Ctrl+C or right click etc). This step must be done with text already in the rich text editor. Text from elsewhere does not demonstrate.
Then in either the same rich text field or in a different one elsewhere, Paste (Ctrl+V or right click, etc).
The pasted text does not appear.
It's there, but invisible. It can be seen by switching to the BB code editor. Return then to the RTE and the pasted text is displayed.
Note I have uploaded the two .js files posted for the 1.5.5a release.