Flashback and syncing

What I did: open the agenda on IphoneX under iOS 12.1.1

What happened: the Agenda flashback

What I did next: delete and download the Agenda from App Store again, the syncing doesn’t complete, and when I closed Agenda using multi-task controller, it flashback again

Can you tell me what you mean by “flashback”? Do you mean the app crashes and you go back to the main screen of the iPhone?

Did you update to the beta version of Agenda on one of your devices?

yes, the agenda crashed on my IphoneX. I have a beta version of Agenda on my Macbook

That explains it indeed, if you use the Mac beta, you’ll also need the testflight beta version of the iOS app. You can register here for an invite: Can I join the iOS Beta?