Sync issues / missing auto-start permission
In most cases when anything does not work correctly on a Xiaomi device, the AUTO-START permission (non-standard extension by Xiaomi) is not set for aCalendar and thus blocks various actions like notifications or tasks sync.
The auto-start permission can be set in Android/Xiaomi settings under Security - Permissions.
For widgets not responding with 3rd party launchers please activate the following setting:
Android Settings - Apps - Manage apps - aCalendar - Other permissions - Display pop-up windows while running in the background
(see here for more details about this Xiaomi bug https://forum.xda-developers.com/poco-f1/help/nova-launcher-widgets-clickable-t3841380)
App Icon Missing
This is most likely a bug in Xiaomi's App Vault, we recommend to simply disable it, see here for instructions: https://android-fix.com/tips-and-tricks/96-what-is-app-vault-on-xiaomi-smartphones.html
Wrong notification icon
If you see the aCalendar app icon instead of the event, tasks, birthday, or date notification icon, please set the notification style in Xiaomi settings to Android (not MIUI).