I haven't been able to place ad breaks manually at all in the YouTube editor for almost an entire month... This has been happening on multiple channels and it's so bad that the page completely freezes up and the whole website breaks. @YTCreators
Have you tried closing & reopening your browser or clearing cache & cookies: https://yt.be/help/Cko1 ? If you're still having trouble, Creator Support is the best team to help with this issue, here's how you can get in touch with them: http://yt.be/help/partners