I can confirm this with my own install of the latest beta. I worked on several notes for a mailing list and found constant sync errors while I was writing. The moment I stopped writing, the syncing stops. I have my preferences set to Manual Sync. After a restart of Evernote with this new preference, the syncing still occur, but only when I am writing in a note. This is not expected behavior when the preference is Manual Sync.
Also, downgrading to an early version is not possible as well, same as @AndyDent writes. Going back to the latest stable release gives me the exact same popup, but