The "Failed to upload files to the storage" error appears if the bandwidth and/or connection is too slow.
Also, this error may appear if desktop or webcam streaming failed.
Here are a few tips that should help you in solving this:
Make sure that the entire screen sharing is on, not just a tab or a window. Check this in Inactive "Share".
Make sure your computer fits the minimal system requirements.
Try connecting to another network and repeat the check.
Close other apps that may keep your webcam and microphone occupied.
Try switching to a wired connection rather than wireless one.
Temporarily disable your firewall and antivirus.
Make sure that your screen can be accessed and is not locked by your system settings:
Are you a Linux user? Then check the guide: Linux: screen sharing\streaming returns black square
If you are taking your exam in a classroom, check with the educational center if the network ports listed here are blocked.