Did you ever unselect the addressbook in the DAVdroid account manually?
This message appears when the addressbook URL has changed (either from one URL to another, or from null to another URL, or from an URL tonull), i.e. the address book URL of the last sync is not the same as the current one. It normally only happens when you select another (or no) address book in the respective DAVdroid account.
If this message keeps appearing, it seems that the sync state (where the last addressbook URL is stored) can’t be saved correctly.
This could be a problem in your Android address book storage, or the synchronization never completes successfully.
So, please provide steps to reproduce, debug info and verbose logs.
Android provides only one address book while it provides multiple calendars. Thus, only one address book can be synced. Creating multiple accounts is an option, but in this case, the user should do so.
The restriction is not by DAVdroid but by Android – there can be only one address book per account.
Using groups would be a possible solution but there are other bugs preventing this (see #48).