Its quite possible that its different. The previous versions checked if the name you were renaming a file to already existed but only at the time it was renaming the one file. The new approach checks for the existence of any of the new names before it starts renaming. Previously what could happen is many dialogs warning you about renaming clashes which were a pain if you happened to have selected a lot of files. The new approach checks at the start and gives you 1 warning.
To work around it in your situation, just rename them all to aChapternn, and then rename them again to Chapternn.
(And I'm glad to see the 'one of more' message is actually 'one or more' in the code
)