According to some post, the nettatalkd build is not a complete AFP implementation, lacking some stuff.
That’s probably why Lion still works with Time Machines that are 2+ years old.
“Apple discontinuing support for DHCAST128 (or DHX) authentication in Lion because it was considered insecure. Instead, the successor of DHCAST128 should be used: the more secure DHX2 user authentication module. DHX2 is supported since Mac OS X 10.2 and supports up to 256 characters for passwords (hell yeah, that should be enough). It relies on CAST-128 in cipher block chaining mode for encryption.”
Rumor has it that some NAS vendors intentionally disable DHX2 in netatalk because it’s a lot more CPU intensive.
Apparently netatalk 2.1.2 supports the DHX2 UAM. What version is Drobo running?
There appears to be a public spec for what is required by a server to provide Time Machine support over a network - Time Machine Network Interface Specification (TMNIS):
The problems seems to be with Time Machine, since I am able to connect to the Drobo using AFP without problems.