@Anne, I notice that John's problem differs from Adam's in that Adam would use the Tech Preview for a few days with each account before being stopped.
John, by contrast, has uploaded to the Tech Preview over a span of 123 days (2014 February 6th - 2014 June 8th http://bit.ly/prof74betauploadspan )
First: http://photosynth.net/userprofilepage.aspx?user=prof74#20 | Last: http://photosynth.net/userprofilepage.aspx?user=prof74
Note that since Photosynth's 2014 June 10 update, John has not succeeded in uploading any new Tech Preview synths.
This is a red flag to me and suggests that possibly some file(s) from the older version of the site may still be cached in his web browsers, which may be creating the conflict with your updated site.
@John, sometime (before adding any photos to be uploaded) try loading the upload page and then hold down the [Ctrl] key and, while held, press the [F5] key, then release both.
This should refresh every file that makes up that page.