>This is my code that doesn't work, maybe you can see the mistake?

well, IMHO one mistake is, that you didn't specify the table where the updates have to go.
So you will need at least this line:

DBSetProp("view5","View","Tables","mydbc!mytable")

Another point is the missing UpdateNames for the updatable fields:

DBSetProp("view5.myfieldname","Field","UpdateName","mydbc!mytable.myfieldname")

>CREATE SQL VIEW view5 AS SELECT &m_selectvelden FROM ALLTRIM(m_dbf2) WHERE SUBSTR(postcode, 1, 2) = >ThisForm.cmbpostcode.DisplayValue

I also would recommend not to use an expression like "...alltrim(m_dbf2)..." in the FROM clause and not using an object reference like "thisform.cmbpostcode.displayvalue" within the SQL statement. But this shouldn't be related to your actual problem.