Windows update error 0x800703ee

Windows update cannot install the Windows 10 upgrade to version 1903, I tried upgrading manually through the Microsoft upgrade assistant and the same error happens, with the error message saying it "couldn't prove it had enough space to install windows" (%) (>25Go disk space available).

Indeed, my disk configuration is a bit weird, having had a lot of problems when switching Windows to the HDD to let my Ubuntu partition on the SSD. Here are the details of my disk partitions :

Current windows version is 1803, build 17134.1006. Any idea what could be going wrong and how to fix it ? Search engines give me no help for this one.

It might not be related, but I noticed a very high disk usage recently, 100% almost all the time.

Thanks for helping

(%) : Rough translation due to my Windows 10 version being in french

EDIT : I freed up >35Gigs and the error is still the same. I'm getting pretty confident this is not a problem related to disk space.

Your disk configuration is more than a bit weird, on these points:

• You have allocated all of Disk 1 to EFI partitions. Why allocate 118 GB to an EFI partition, when its size is at most 500 MB?
• Disk C is relatively small for Windows 10 and the 17 GB that are free might not be enough for the upgrade.
• I do not understand what purpose is served by the 109.32 GB partition (this might affect the rest of my answer below).

My advice would be:

• Reduce other partition(s) on Disk 0 to:
  • Add a few dozens of GB to C
  • Leave at least 500 MB as unallocated space for a new EFI partition
• Upgrade Windows 10 to version 1903
• If the above unallocated space is now used for a new EFI partition, you may now reformat Disk 1 and use it for data.

Before starting, I recommend taking an image backup of the entire Disk 0. A product I recommend is AOMEI Backupper Freeware, and for good measures create also its boot media and try it in order to ensure that it can see the backup device, just in case.