Damn, now I'm ashamed of myself - it's since forever that I'm using rsync... :)

I think that when I tested this with rsync a long time back I did a similar mistake like this one https://stackoverflow.com/questions/28819379/block-level-cop... (doing a sync not over a network) and/or this one https://www.reddit.com/r/linux/comments/3nhx0p/rsync_block_l... (doing a sync by adding/removing some bytes instead of just changing their values, as it would happen in the preallocated img-file of the VMs).

I'll test it again - thanks a lot for the info!!