Authenitcation is done based on schema and authority component (e.g. FQDN) of the URI and the realm (see RFC 7235, Section 2.2)
If a user wants to access multiple accounts via WebDAV, neither schema nor FQDN will differ. So the realm has to differ for different acconts. Currently the realm is a the fixed string "KolabDAV".
Solution is to add the email address, extracted from the URL, to the realm. This allows Clients like Thunderbird to access mutliple address books and calendars from different accounts on the same Kolab Server.