If it required adding youtube fields again before attempting upgrade they should have said that on the 3.3.9 release post.
This is what it says on the 3.3.9 release post
Well, it is not resolved unless you manually add the fields before attempting upgrade. Why not say that upfront to avoid people going through this error and reporting?In addition to this, we have also resolved an issue that was preventing users from updating after having removed the default YouTube profile field. Furthermore, some issues with malformed emails due to special characters in usernames as well as differing behavior related to line endings in newer PHP versions were fixed.