Since the problem has shifted to Synology 1815 because of the “Out of Sync Items”, I’ll give a brief outline of the things.
The allegedly not synchronized elements (10.391) should be contained in 6 peers (folders):
However, the individual peers are all up to date and do not show any “Out of Sync Items”:
And yet this deviation is still displayed today:
Why are the 10.391 elements not divided and displayed among these 6 peers?