Hello Sabine.
I hope that you have not yet put into practice the "good advice" from this forum to simply apply any filler on a wooden substrate that swings at a depth of up to 1 cm?!!
Because that does not work, even without load (meaning in this case: without use)!
Laminate floor elements, as they are correctly called in technical terminology, require a level and pressure-stable substrate.
Level in this case means that when a straightedge is laid on the floor with and without pressure load over a distance of one meter (between the front and rear support points of the straightedge), only 3 mm, maximum 4 mm, of deviation per meter may occur underneath the straightedge.
Applying filler means increasing the scope of the later renovation by the amount of dismantling again.
Apart from the fact that a wooden surface must first be sanded, the dust vacuumed, and primed before a filler advertised for wooden substrates can be applied in a thin layer.
What happens if you proceed as "recommended" in this forum?
Well, an unsuitable filler (e.g., for mineral substrates) would detach itself from the substrate during drying due to the inevitably occurring tensile stresses in the structure. Visually, cracks would appear on the surface, and at the slightest foot traffic, the structure breaks down, and you can sweep up the broken pieces of the crumbled filler with a broom.
At the latest when deflections in the mentioned size occur underfoot, the surface will collapse in on itself.
Even in the hypothetical case that the filler could withstand these deflections, and the laminate is laid "floating," openings in the locks would inevitably form due to the deflections between the laminate floor elements. In summary, this means that when walking on the laminate floor elements with partially significant gaps between the long edges and end joints, the filler crunches, indicating that it has "crumbled."
A complete renovation would then be the consequence.
The correct way would be either to remove the existing boards and level the substrate so that it is pressure-stable enough to allow installation, or better, to lay a vapor-permeable layer on the existing boards and stabilize the entire surface with 22 mm thick OSB boards so that only Spax screws are used (these have continuous threads. Otherwise, it can "creak" later if they loosen by just half a millimeter).
All other solutions, where apparently something is quickly and consumer-friendly applied on the existing substrate, are doomed to fail!!
Good luck with the future renovation: Kla Ra