Thanks! I ran both.
Weird:
TreeSize reports 257,2 GB with a huge folder System Volume Information. It contains 185 files, together 98,9 GB.
Names like: {710bf832-b3e2-11e1-b5cf-206a8a2014f1}{3808876b-c176-4e48-b7ae-04046e6cc752} to which acces is denied, probably system restore files?
The 98,9 GB seems to count for the extra GBs I see in Defraggler and C disk properties.
WinDirStat reports no obvious things, gives the original diskspace usage (158,4 GB). The folder System Volume Information is reported completely empty!