This error means Team Explorer was unable to find any of its plugins, which are discovered via MEF. Running as Administrator should not be required.
The first action we should try is to close all VS instances, open a Developer Command Prompt as administrator, and run 'devenv /setup'. This will rebuild the various caches.
If this does not correct the issue, the next step is to try manually deleting the MEF cache dir. Please delete "%LocalAppData%\Microsoft\VisualStudio\15.0_\ComponentModelCache". The cache should be rebuilt on the next VS startup.
Regarding "Manage Connections" doing nothing, this was indeed a bug in the VS 2017 RC3 release. This has been fixed and is pending release.
For reference:
https://developercommunity.visualstudio.com/content/problem/17942/team-explorer-does-not-work-at-all-just-shows-page.html