I have another similar issue now...
So, after all the issues I faced syncing as discussed above in my previous posts, I decided to remove my security keys. Which wasn't ideal, but nevertheless I did it for compatibility sakes.
I recently turned on "Advanced Data Protection" from my iPhone so that all my data including iCloud sync data has end-to-end encryption (which is something I really wanted) it works well with the Diarium App on iPhone. Unfortunately, the Windows version of Diarium refuses to play nice with this feature too!
I created a journal entry on Windows Diarium hit the sync button and I was presented with the log-in screen for iCloud.com I filled in my log in information, completed the standard 2FA procedure and the screen disappeared as it usually does signifying that I am now logged in. However, when I hit the sync button, the iCloud login screen appears again! so I fill in my details again, hit submit, again, the window disappears, then when I press sync, the "Diarium Sync" window appears asking me to chose a sync provider!
Anything to do with security and syncing does not work well with windows - this is most frustrating. I've attached the screen shots for reference.