First of all I have to say that I absolutely LOVE both FEBE and TEBE!
I just installed the updated version of TEBE (20100727-120000) because I upgraded to Thunderbird 3.1.2 on openSUSE 11.3 GNOME. I recently set up a Box.net account so I decided to set up online backup through TEBE (works fine in FEBE).
For whatever reason, TEBE wouldn't take my password to login to Box.net. What I mean by that is that my password consists of 10 characters, which I entered into the appropriate box and clicked Save Login. Then when I click on the Test Login button I get the Login Invalid error. Furthermore, upon closing the TEBE options window and opening it up again I find that the login fields are grayed out but that there are only five *s in the password field (remember, my password has 10 characters). Obviously, trying to login again ends with the same results. Again, no problems with FEBE when I set it up.
I managed to fix this by tinkering with the Config Editor. I found that the extensions.tebe.boxpassword entry was unaltered and empty. I double-clicked on that entry and entered my password in the box that opened, and now when I return to the TEBE options window there are 10 *s in the password field (still grayed out) and the Test Login succeeds.
This is obviously a work-around, as I'm guessing this is really a bug.