unidentified files following upgrade to Win 10 version 2004 prevent defragmentation

I have recently upgraded my Win10 x64 PC to version 2004 from Windows update. Prior to the upgrade the PC was clean and fast and the system drive was defragmented using Auslogics Defrag Ultimate. Fopllowing the upfgrade and subsequent cleanup, I ran defrag again and the drive now has approximately 20% of its used capacity filled with files that Auslogics Defrag will not identify (light green block icons, mouse click for file info invokes the prompt "This block contains file system service data. Detailed view is not available".

This is obviously some several GB of data judging by the number of blocks occupied in this manner - but I am unablæe to find out what these files are, what they are called, what purpose they serve or where they are on the drive. They are spread out across the system drive and thus prevent defrag running a free space defragmentation, and thus casue even more fragmentation when new files are added to the drive.

I have tried tio google this problem but have not found any reference to it. Auslogics defrag is the only untility that tells me these file exist - but no more than that.

Can you advice - what are these files, and how do I get rif them and run the usually perfect free space defrag?

thanks / Adrian