Hey Michael,
What’s the details on the Dev server? Is it a sub-domain of the main site? Are you running ExpressionEngine in both locations?
You very likely need to set an explicit Cookie for your dev site.
Admin > Security and Privacy > Cookie Settings: Cookie Domain
Set that to .dev.domain.net - using the proper values, of course.
I suspect EE is mixing your live and dev sites with a cookie set for the main domain, which is being invalidated when the Control Panel times out.
Cheers,