Calling all QuickBooks Pro 2008 invoicing system users

This software has it's fair share of bugs (I keep thinking I'll ask Tom to code us up a different invoicing system instead, but our accounts dept have spent a while getting all our clients details and templates etc in there now), but by far the most annoying one is when it suddenly decides our accounts person doesn't have permission to use it on their PC. It's the only computer we've ever installed it on, yet every so often it'll refuse to open, saying they don't have the correct license etc. But then a couple of days later they'll try again and it will be fine. But that's no good if we need to raise an invoice today! And we do, now! And yes, I know it's not expired - we haven't had it a year yet. Any ideas/experience gratefully received!