I get the 'zip file is empty' error, on the first download. The Dutch versions of Access apparently does use the semi-colon instead of a comma, and since the Table only has a single Record, all you really need is
=DLookUp("[OMnummer]";"Administratievegegevens")
The Table name, because it's a String, and in Quotes, doesn't need the square brackets; doesn't hurt, but isn't necessary, as it would be if you were talking about a Field Name.
As has been said, number is a Reserved Word, and a poor choice for a Field Name, but once again, it shouldn't be causing a problem, here...didn't when I tested it.
I'd try the above syntax, without the Criteria, and see what happens.
Linq ;0)>
The problem with making anything foolproof...is that fools are so darn ingenious!
All posts/responses based on Access 2003/2007