From looking into this a bit today, I can think of two possible reasons you are having this error.
Can you verify the version of the storage service that you have installed? From the command line, you can type:
aptitude show archivematica-storage-service
and it should show you information like:
Package: archivematica-storage-service
State: installed
Automatically installed: no
Version: 0.3.0.release
If you see a different version in there, you may have installed the storage service from the daily PPA, not the release PPA? That is one possibility. The daily PPA has test builds in it, there could be (and in fact definitely are) bugs in those builds.
Another possibility is that you have used an email address as a username for one of your storage service users.
We discovered a bug today, that appears to be happening in the Django web framework used by the Storage Service, where usernames with a dot or an @ symbol in them can be created, but they cause an internal server error when displaying the list of users. We have a fix for this that will be in the next storage service release (0.4.0).
In the meantime, if you have a username with a dot or an @ symbol, you will have to fix it from the command line.
You may need to do this as root, check the permissions on the /var/archivematica/storage-service/storage.db file and make sure you do these steps with a user that has permission to write to that file.
cd /var/archivematica/storage-service
sqlite3 storage.db
select id, username from auth_user;
You should see all the storage service users. If any of the usernames have dots or @'s, change them with a statement like: