Hey @renan,
Thank you! It seems to have fixed the issue.
Please take into consideration, I don’t think that this information was posted anywhere, and it’s very misleading (it actually makes no sense).
Take a look at the scenario:
Key A - My CF key
Key B - Customer’s CF key
- My customer added me to his CF account as an admin.
- I have used MY key (Key A) to upload the videos through the API - No problem!
- I generate links using MY key (Key A) to a video without “require signed url” - No problem!
Problem:
4. I generate links using MY key (Key A) to a video WITH “require signed url” - doesn’t work, and no specific error on the response from CF
No problem:
5. I generate links using my CUSTOMER’s key (Key B) to a video WITH “require signed url” - Works!
Now, I hope you see where the issue is - I can do everything with my key except generating urls for videos with “require signed url”.
Note - the videos were uploaded with KEY A, but can only be viewed securely by KEY B.
From my perspective it’s a bug, as I would expect not to be able to generate links at all, and even not upload the videos… but when one function out of everything doesn’t work, without any specific error, and I can’t even find any explanation for this in the documentation - it’s a bug.
I hope CF will take this into consideration, as this cost us a lot of debugging time, and definitely me in frustration and stress-choclate-eating 
In other words - thank you so much for your insights, and I really hope this would be more clear on the documentation (or fixed if you consider it as a bug like I do), so other customers wouldn’t need to find this as well.
Cheers!