InvalidXMLResponce when syncing with Baikal

When starting vdirsyncer discover with all the entries in the config-file and using for url = "https://baikal.xxx.de" I'm getting the following error:

Discovering collections for pair my_contacts
my_contacts_local:
warning: Failed to discover collections for my_contacts_remote, use `-vdebug` to see the full traceback.
error: The server returned something vdirsyncer doesn't understand. Error message: InvalidXMLResponse("Couldn't find home-set.",)
error: While this is most likely a serverside problem, the vdirsyncer devs are generally interested in such bugs. Please report it in the issue tracker at https://github.com/pimutils/vdirsyncer/issues
Discovering collections for pair my_calendar
my_calendar_local:
warning: Failed to discover collections for my_calendar_remote, use `-vdebug` to see the full traceback.
error: The server returned something vdirsyncer doesn't understand. Error message: InvalidXMLResponse("Couldn't find home-set.",)
error: While this is most likely a serverside problem, the vdirsyncer devs are generally interested in such bugs. Please report it in the issue tracker at https://github.com/pimutils/vdirsyncer/issues
error: 2 out of 2 tasks failed.

With -vdebug the lines before the first warning are:

debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/cli/discover.py", line 204, in _print_collections
debug:     discovered = get_discovered()
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/cli/discover.py", line 133, in get_self
debug:     return self._discovered
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/utils.py", line 170, in __get__
debug:     obj.__dict__[self.__name__] = result = self.fget(obj)
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/cli/discover.py", line 142, in _discovered
debug:     return handle_storage_init_error(self._cls, self._config)
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/cli/discover.py", line 138, in _discovered
debug:     discovered = list(self._cls.discover(**self._config))
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/storage/dav.py", line 251, in discover
debug:     for c in self.find_collections():
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/storage/dav.py", line 211, in find_collections
debug:     return self._find_collections_impl(self.find_home())
debug:   File "/usr/lib/python3/dist-packages/vdirsyncer/storage/dav.py", line 198, in find_home
debug:     raise InvalidXMLResponse('Couldn\'t find home-set.')
warning: Failed to discover collections for my_contacts_remote, use `-vdebug` to see the full traceback.

vdirsyncer 0.16.2
Baikal 0.5.0
Python 2.7.15rc1 and 3.6.7
Ubuntu 18.04

Thank you for your help. I would be very happy if it'll work!

Update: it doesn't work with url = "https://baikal.xxx.de//card.php/principals/xxx", with just one / after the domain or with only card.php.

Start in vdirsyncer/storage/dav.py at find_home(), _find_collections_impl(), and discover(), then reproduce the failure with the Baikal configuration and -vdebug. Compare the response handling around the reported “Couldn't find home-set” error; done when the Baikal discovery behavior is understood and the issue has a verified resolution or a clearly documented incompatibility.