I'm using a water pump for an undergravel filter with 300 l/h (Eheim universal 300). If the filter is blocked because or irregularities, the pump starts to suck air because water isn't sinking fast enough through the gravel. Inserting a valve and limiting the input of the pump causes it to run smooth/without noise and without sucking air. Can this input reduction be somehow harmful for the (lifetime of the) pump?
Short answer, Yes it can.
Your product link is not English, and I did not see diagrams. Different designs can have different issues. Less water is probably better then sucking air, so your solution is likely better for the pump then without it.
The pump is designed and tested for moving a specific volume of water. When you limit this there will be strains on the pump.
Instead of inserting a valve to the input of the pump, you should have another (a bit more complicated) setup.
Add a "short-circuit" between the input and the output, and insert the valve in the "short-circuit".
If the valve is completely closed, then the pump works as designed.
If the valve is (partially) open, then some water will flow through the "short-circuit", therefore reducing the flow through the pump, and through the tubes (especially the exit one, which is of interest to you).
This setup should have no negative impact on the pump - but it might use more electricity compared to a smaller-sized, suitable, pump.