We are aware that there is an issue with authenticating to YouTube. We have notified YouTube in hopes to have this resolved soon.

In the meantime, we do have a workaround to export your project file as an MP4 file and upload the MP4 to your YouTube channel using a web browser, please see link below of a knowledge base for this workaround. 

http://telestream.force.com/kb/articles/Knowledge_Article/ScreenFlow-Upload-to-YouTube

Thank you for your patience while we work through this with YouTube.