publish ebook to Apple created with pages on iOS failing

Hello,

i've got some weired issue with Pages and its option to publish a writing directly as ebook to Apple Books.

I started writing my book with Pages and the "empty book" template on iphone and my iPad. Everything worked fine, the for testing purpose exported epub file looked well in Books App so i decided to push the Publish to Apple Books button, logged in with iTunes connect, filled in missing informations like category etc., and then pressed upload. After a while "uploading" i got the message {"error";"No data found"} :-(

I don't know what this error means, and where my mistake may be? Again, the imported epub file works perfectly with Books app so i don't know what i could do?

I tried to upload the epub from Pages export with the old Apple Books Authors page but that failed to. This time with a "no validate possible" error :(

My question is, what could went wrong, and why does Pages produces for Apple Books unusable epubs? For the record, i did not used any pictures or 3rd party apps to create the content (and it is a text-only book).

Any ideas anyone?

Thanks for help in advance :)