Well, probably the same unsatisfying answers you don't want to hear: Just keep trying! OS X is getting more screwy with every release and so is AE. Upgrading therefore might not solve anything because neither Apple nor Adobe have got their act together. Perhaps you have outdated entries in your Keychain that block network stuff, perhaps you need to scrap your network and rebuild it from scratch, perhaps it's just some weird permissions issue on a single file somewhere. Impossible to know. At least try to create a new user and run AE from there, possibly also reinstalling it under this new account.
Much respect Mylenium... I have gained much knowledge from your tutorials. This is pathetic that Adobe hasn't solved this problem. Bottom line is, who has time to dig into all these possible solutions when you're working on a job with a deadline. Thankfully, I still have Camera Tracker working on my old laptop system.
I'm not sure what "Outdated entries on the Keychain" is but i'll look that up.