Compliance of a pdf generated by Scribus with Amazon KDP requirements

[postal code]

Hy to Scribus community,
I am publishing a book in paper format on the KDP Amazon distribution platform but unlike previous occasions, this time there are problems with the pdf I uploaded.
First set the Scribus document to the correct size corresponding to the required one (8.5x11 inch) as well as activated the color management and the prerogatives of the pdf / x-1a format.
Tried both font embedding and their outline, but after loading the site always replied that there were problems (without specifying which ones).
Has anyone using this publishing medium found themselves in similar situations? How did you resolve the stalemate?

[toyotadesigner]

Did you convert all RGB images to CMYK?
Does the KPD distribution platform offer PDF/X-3 or PDF/X-4? If yes, you don't need to convert the images.
Font outlines increase the file size, embedding is always the better option.

Too bad they don't specify the bug...