Open your ePub with an Editor such as BBEdit and switch to the content.opf file.
Starting the second line you need to search for »rendition: http://www.idpf.org/vocab/rendition/# « (including the space at the end) and delete it.
Just save and go on with your epub. Now it should validate again. With InDesign CC 2015, this has been fixed. It was an amendment, the IDPF has introduced recently with the 3.0.1 update so all previous applications didn't know, that this is not allowed anymore in epub. epubcheck has much evolved. There have been many new parameters integrated to check. Unfortunately it has become slower and many old validated ePubs are not valid anymore. I wonder how ePub distribution services now handle this situation.
My German followers can read the solution on my blog: https://ebook-world.eu/e-books/epub-3/epubcheck-4-erschienen
Thank you so much for your answer!
I found your website and tried your method earlier today, but it didn't work. Maybe because I unzipped or zipped the file not in the right way. But you say that this problem is fixed in Indesign Design CC 2015, but I am working with ID CC 2015 and the epub was made today. All the previous epubs I made which had no warnings or errors before, have now the same warning with the Epub Validater 4.0.0.
I will try again and let you know if it worked.
I made a second test. Indeed, it's not fixed for fixed layout in CC 2015. Its fixed for reflowable only.
[EDIT]: Its fixed for reflowable. However, I'm not sure at all that it is an error in fxl: http://www.idpf.org/epub/fxl/#usage
So it might be also the case that epubcheck is wrong. I will open an Issue at Github.
I just openend the file with BBEdit en it worked!
Thanks a lot!
I hope it is an error in the Epub Validator 4.0.0. , cause otherwise all the earlier made epubs are having the same problem.
The Github Issue can be tracked under the following address:
The developer considers removing this warning message again. But things may take time. Until then, you probably need to remove this entry yourself.