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This was my first guess then I read again your comment: Northern Poland. Lysimachia vulgaris is usually found on the South Eastern Europe.
But you saw the plant, I just watched some great photos. Vulgaris it is. 🙂

Lysimachia vulgaris
Lysimachia vulgaris, the yellow loosestrife or garden loosestrife, is a species of herbaceous perennial flowering plants in the family Primulaceae. It is native to wetlands, damp meadows and forests of south-east Europe. It is a tall plant with an upright habit, 50–150 centimetres (20–59 in) high, with erect panicles of conspicuous yellow flowers. The edges of the petals lack the fringe of hairs seen in L. punctata, and the hairy sepals have a conspicuous orange margin.