Did you figured it out?
I have the same issue with Maxtor Central Axis NAS. I have no problem to connect to other NAS drives in my network as well as I have no problem to connect to Maxtor from other devices and PCs including XP, Win 7, Linux based Maemo (Nokia N800) and Patriot Box Office media player which also has some sort of Linux inside.
Besides Estrong, I tried Astro and another file manager (forgot its name) but all of them give me authentication error. I start to think this is Android system bug. Looks like it has problems with some smb shares.