I stopped syncing LR Classic and LR CC for a month. In the meantime I deleted some 5000 images from LR CC. Now I have initiated sync again but it it seems not to work. The 5000 images which a deleted from LR Classic seem to remain in LR CC. What to do?
Check where you deleted the images. Synced images are in a synced collection, but removing those images from that collection does not delete them from the catalog and they could still be in 'All Synced Photographs'.