Could not add rule to fixup DHCP response checksums on network 'default'ΒΆ

This is a warning message, and is almost always harmless/innocuous, but is so often erroneously pointed to as "evidence" of a problem that it warrants a separate entry here.

It is more fully explained in PXE boot (or dhcp) on guest failed, but in short - this is almost always a classic "red herring"; unless the problem you're experiencing is guests that are unable to acquire IP addresses via DHCP, this message can and should be completely ignored.