Hi @Nicola Marconi,
I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to accept the answer.
Issue: The database restore process is failing through all available options. When attempting to restore on a SQL VM, the instance is not detected, resulting in an error indicating that not all required fields are populated. Restoring to a file is also unsuccessful, and restoring to the original location fails as well.
Solution:• As you stated, it was a temporary issue with the Microsoft Azure Portal.
If I missed anything please let me know and I'd be happy to add it to my answer, or feel free to comment below with any additional information.
Please remember to "Accept Answer" so that others in the community facing similar issues can easily find the solution.