What is this plant? The white flower clusters reveal an old garden favourite

The plant is border phlox, Phlox paniculata, apparently a white-flowered cultivar. The clues are dense clusters of many small, five-parted flowers, long narrow flower tubes and opposite, lance-shaped leaves.

Flowering well in September in Norrmalm, Sweden, also fits border phlox, which often continues from late summer into early autumn. If you want to keep or move it, dividing or replanting in spring is better than late autumn, giving it time to establish properly.