When you activate 2FA external app like thunderbird can't connect anymore with basic password. It's quite normal from a security point of view. The problem is that's not clear before you launch your calendar or contact syncing app and you see it doesn't work anymore

Over the activate cas of a 2FA system, a red warning should inform you that you MUST use an app password before activating the 2FA system. Especially if the system is using an app on external device like Notification. In the same idea it should warn you to creat and save backup code before even it look like the Android apps credentials aren't compromised by the 2FA activation so you shoud not be locked outside. DavDroid is concernet but it's easy to change password without need creating a whole new account.

Just an enhancement that could save frustrating time ;)

Have a nice week !