bb-drummer, I don't think your problem is related to the error in this topic but here are a few details that might help.
You don't really need to define an 'ID' field and index on your DataObject since this is created automatically.
These don't seem necessary unless they are adding new fields to the dataobject
class Bereiche1Mitglieder extends BereicheMitglieder {}
class Bereiche2Mitglieder extends BereicheMitglieder {}
class Bereiche3Mitglieder extends BereicheMitglieder {}
then you can just use:
public static $has_many = array(
"Bereich1Mitglieders" => "BereicheMitglieder",
"Bereich2Mitglieders" => "BereicheMitglieder",
"Bereich3Mitglieders" => "BereicheMitglieder"
);
on your DataObject also you could maybe just use:
static $has_one = array(
"BereicheMitgliederPage" => "BereicheMitgliederPage"
);
on your page object, you probably don't need these line too:
$itemsInGrid1 = DataList::create('Bereiche1Mitglieder');
$itemsInGrid2 = DataList::create('Bereiche2Mitglieder');
$itemsInGrid3 = DataList::create('Bereiche3Mitglieder');
try this and see how it goes. Also would be good if you could give us more details on the error returned.