In OBS the build of php-sabre-dav-2.1 for Debian9 in Winterfell is disabled:
https://obs.kolabsys.com/package/show/Kolab:Winterfell/php-sabre-dav

In the debian repository there is still a package for php-sabre-dav-2.1:
http://obs.kolabsys.com/repositories/Kolab:/Winterfell/Debian_9.0/

If we can live with stock debian package, we should remove the Winterfell package.