I found this nice looking wasp on the bee hotel, it is a female potter wasp Ancistrocerus nigricornis .
She was busy building her nest in one of the nest blocks in section II of the bee hotels.
After the interior had been prepared she flew off and on with prey caterpillars, with approximately fifteen minute periods. The wasp stuffs a number of five or more caterpillars into each chamber depending on the size of the prey. This pdf on bijenhotels.nl shows a good view on the inside of a nesting chamber on page 313.
These wasp create nesting chambers in the tunnels, just as bees do. In a chamber an egg is laid together with a number of prey. In this particular case the wasp flew to and fro to bring balls of clay to the nest.
When the cell has sufficient provisions it is closed with mud which the wasp will collect and transport to the nest in balls.
After the last chamber is filled up the tunnel is closed.
References1 Determinaties muurwespen (Ancistrocerus) AHW