I have now narrowed down the error with the Default MX to the dns_dep table. dns_mx and dns_nodes are correct but the corresponding entries in dns_dep (child_doctype_id=1020) are missing. In other words, the SQL query in ispconfig_bind.lib.php at line 210 which (in my case) is:
select dns_mx.host, dns_mx.prioritaet, dns_mx.mailserver
from dns_dep, dns_mx, dns_nodes
WHERE dns_dep.parent_doc_id = '10' AND
dns_dep.parent_doctype_id = '1016' AND
dns_dep.child_doctype_id = '1020' AND
yields no results, already taking only the first 3 conditions into account.
Some previous script must have forgotten to create these entries. Can someone take a look?