It sounds like maybe you didn't get all of the files applied during the upgrade.  I would check the upgrade zip package and make sure it is unblocked (Windows will block zip packages downloaded from the internet which results in some of the files not getting upgraded when you unzip the package).

Assuming that is not the problem, I would use a proxy like Fiddler to see what url is getting called during the file drop down and then troubleshoot the issue from there as Fiddler should show you where the file request is failing.