The two notes that I currently can't get to merge are less than 6 months old. Other notes, some more than three years old, merged just fine. These two notes have HTML content, but again, I have others with HTML content that merged with no issues.
Still having this error as well, and I am on the latest version. 10.6.9 on a Mac.
Getting two different errors - in some cases, the app crashes. In the second case, I get the red banner notification that they were unable to merge the notes and please retry.