Thanks for the help!
Unfortunately I have already spoken with my supplier, which is not able to provide me with this part here in Germany.
I did some research, according to the UM3 Bill of Materials, the missing part should be a "Threaded Insert Multisert 004M3" (Part No. 2867 B).
I think i found the insert at the manufacturers page (see screenshot), but I only found b2b resellers in larger quantities.
Do you know if there is a supplier (globally?), where I can buy the part from?