Sorry Gaël, I don’t have an answer for you. I did what Anshu Kumar said and for me it worked. Also I don’t use Windows system, all my devices are MacOS / IOS based. But I do have a suggestion that I hope can help you somehow. Connecting to iCloud dot com with a web browser and going to iCloud Drive page you should see the Enpass directory, do you have a walletx file inside? I have two. The OLD and the new one. Have you tried to download it to your computer and maybe adding the data from there to then create a new password database (without losing what you already have) . I’m not sure this works, I never did it, since for me what worked the was the suggestion from Anshu Kumar. But maybe it could help you. Another suggestion would be ask directly to Apple since this might be an authorization problem and probably they could help you a bit more from their end.
I hope this can help you somehow, sorry if I can’t be of more help.