5.0 Total Backend Lockout

Lucky for me my production Server doesn't do this.
    Ser 2008, II7, SQL Server 2008 all with current updates.
I wanted to do an update from 2500 release.
If I pull the files and Db back down to Dev server, build a completely new site following classic SF processes accessing the back end is now impossible. Yes the Dev Server built the prod server less than 45 days ago. ONLY the Db has changed.
    Dev Server Win7, II7.5, SQL Server 2008 all with current updates
Logins are authed in the Db tables BUT cannot access the backend pages at all.
Classic sit on the login page FOREVER behavior covered in TOO MANY threads here.
Using FORMS NOT CLAIMS.
Web config for 4.4 upgrades is GOOD according to RECENT posts in other TREADS.
NOT COOKIES, NOT BROWSERS, not bad page redirects.
Anyone with another idea?

I have no idea, but FWIW: I've had issues with upgrades not fully completing if run while the IIS site was live, and resulting in some files in the site not being updated. I've found that the only reliable way to update is to shutdown the IIS site first, then run the upgrade, then restart the site. This is obviously only an issue if running the development site on IIS.