I tested c4d.PrefsLib_OpenDialog function in Console as well as in Script Manager. I tried to pass almost all pages as its Parameter, but it opens only the Interface Page.
Thanks for reaching us, I can indeed confirm the issue and I've forwarded it to our development team.
Edit: It seems that symbols are wrongs atm.
So you need to pass the actual plugin ID of the correct pref.
You can find the correct ID with this line of code, simply change "Memory" by the one you are trying to open.
[plug.GetID() for plug in c4d.plugins.FilterPluginList(c4d.PLUGINTYPE_PREFS, True) if "Memory" == plug.GetName()]
You solved yet another issue.