I have a test instance of Prosites setup on my server... I create a new child site and then try to do the upgrade of the site and it won't let me login to the parent site to complete the upgrade...
Here is a video to show you this and all my settings:
Thanks for any tips on how to debug this... this is all brand new installed...
The list of activated plugins on the parent site are:
New Blog Templates
Pro Sites (Formerly Supporter)
SG CachePress (Siteground caching) - I disabled and no change
WPMU DEV Dashboard