After the first have_rows() loop you no longer need to supply the post ID
// post id not needed in sub loops
if( have_rows('codegroep') ):
while( have_rows('codegroep') ) :
Other than this the only reason I know of that the row cannot be update is that there are not rows to begin with and have_rows() is false. You cannot use have_rows() loops to update repeaters that have no values/rows.
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