Hey I just thought I would share this, because I just learned of it myself.
Apparently with OpenSim version 7.5 localization has become important, meaning the location given by your server for your Robust instance.
You have to explicitly reconfigure the system locale to en_US then reload them to get OpenSim to allow login's to happen with Linux Servers.
I was running Ubuntu 10.04 on a GoDaddy Server and was banging my head against a wall of why I was getting "Unable to authenticate avatar" even though I was sure everything was correct. As it turns out if the Ubuntu Linux's locale is not explicitly set to en_US by using
"sudo dpkg-reconfigure tzdata" then selecting US and a Timezone.. then by
"sudo locale-gen" to reload...
...your OpenSim 7.5 install won't play nice and let any user (console created or Jopensim) login...
Just a tip! Love the work Foto!