hpetter,
I can see that you are using the embed video link for your videos, such as https://youtu.be/zYCHLTp4f9I rather than the youtube page link, https://www.youtube.com/watch?v=zYCHLTp4f9I&feature=youtu.be. It seems that the Magnific popup that we are using now can only handle the youtube page links and not the embedded links.
The solution would be to change the video URLs. Either edit the database directly, such as using phpMydmin. You will find the video URLs in the postmeta table with the meta_key of maxgallery_attachment_video_url. For example, change https://youtu.be/zYCHLTp4f9I to https://www.youtube.com/watch?v=zYCHLTp4f9I. Or if you prefer not to edit your database, just delete the videos from your gallery and readd them with the using their youtube page URL.
If this solves your issue thanks in advance for marking this thread as ‘Resolved’ so we know your question has been answered and for giving us and our plugin a rating at Maxgalleria Reviews
Alan