Uploading book from Lightroom to blurb fails

Dear all, 

I was used to create books (240 pages) from my old imac in lightroom 6 and print them with Blurb. 

But this is the first time i try this operation with my new configiration : Apple Mini M4 + Lightroom classic., all pictures beeing shot with a Canon R5.

And it went wrong.

1. When i try to upload my new book (240 pages) in Blurb, it fails on the "cover page" upload step.
2. When i simply try to export the book as a PDF file from lightroom, it works and i can read it perfectly in AcrobatReader. (Cover page file and all pages file). 
3. When i try to upload this pdf file via the Blurb website, it also fails with unexplicit error (just asking me to call my support line)
4. I tried to upload some subsets of my books. It worked once for a subset that included the cover page and the first 45 pages, but it is not systematic. Now i cannot upload anything (even a very simplified 20 pages book with no text, no legend, just pictures and a cover page). It always fails on the cover page uploloading step.

I am lost. Does anyone know what configuration i should change to fix this ? 

Thank you very much for your help !