Kimberly,
So this is a new 2.7 install? Did you ever get the screen where is asked you for your portal url?
Sorry, should have given more info in original question. The error was from startup on 2.6 to download an app before beginning the upgrade to 2.7.
Kimberly,
Did you install 2.6 and 2.7 in different folders on your machine?
I keep all versions in folders that are housed in the same folder.
OK, I was wanting to make sure you did not attempt to overwrite the same folder. Hmm... So your 2.6 worked fine until when?
It worked fine last Friday when making a few changes.
My Firewall is off and we've had no changes to anti-virus since then - when googling I found this error in other applications can be related to those sorts of issues.
Have you looked in the WAB appbuilder.log?
[install dir]\server\logs\appbuilder.log
Just did, there is nothing from today in there and nothing unusual from my last log in.
Very Strange. But it is not necessary to be able to open the 2.6 version in order to upgrade to 2.7. You seem to be using the manual method of upgrading apps by importing them one by one from the exported zip. The better way is to use the cmd line upgrade ability.
Upgrade apps—Web AppBuilder for ArcGIS (Developer Edition) | ArcGIS for Developers