Redmine

Hi everyone.
We consider using Redmine as our internal bug tracking solution, but we've bumped into an issue:
Whenever a user logs into the redmine, he is memorized by IP address. Since we're behind a router and redmine is hosted externally, we constantly jump into each other's sessions. Is there a way to fix this?
If these quircks are not related to our single IP address, I'm clueless then.
Please help out.

Best regards,

Valentin.