I'm seeing the same, but only when LM's "Enable belt stacking" startup setting is set to anything other than the default "Separate Stack Loader". The stack loader only exists in that default setting but AB tries to reference it no matter what.
Fortunately, the fix was pretty quick. In AB's data.lua, line 281, change
next_upgrade = "stack-mdrn-loader",
...to...
next_upgrade = (settings.startup["mdrn-enable-stacking"].value == "stack-tier") and "stack-mdrn-loader" or nil,
...and that's it.