I had to realize that the AnonymizeIP plugin is not 64bit clean. It sets either 255.255.255.255 or 0.0.0.0 as IP address, depending on the setting of ip_address_mask_length.
The best workaround I found was masking the result of the anonymization with 0xffffffff, as you can find in the attached patch.
(In ) fixes #1735 - thanks arnotron
(In ) refs #1735 - oops; my mistake