Does this user have permission to access the GAL?
If not, the options are:
a) give the zimbra user access to the GAL
b) configure the addon not to ask for the GAL (Thunderbird Tools/Zindus/Edit)
If that doesn't explain it, have a look in the
logfile - very likely the explanation will be there, or if you can't make sense of it, file a
bug report.