Error after upgrading to revision 5, can't downgrade to revision 4

After upgrading cloudsupport from revision 4 to 5, the unit ended up in the error/idle state, with the following error:

Traceback (most recent call last):
  File "./src/charm.py", line 8, in <module>
    from lib_cloudsupport import CloudSupportHelper
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/lib/lib_cloudsupport.py", line 16, in <module>
    from openstack.exceptions import SDKException
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/openstack/__init__.py", line 17, in <module>
    import openstack.config
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/openstack/config/__init__.py", line 17, in <module>
    from openstack.config.loader import OpenStackConfig # noqa
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/openstack/config/loader.py", line 28, in <module>
    from keystoneauth1 import adapter
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/keystoneauth1/__init__.py", line 16, in <module>
    __version__ = pbr.version.VersionInfo('keystoneauth1').version_string()
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/pbr/version.py", line 500, in version_string
    return self.semantic_version().brief_string()
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/pbr/version.py", line 493, in semantic_version
    self._semantic = self._get_version_from_importlib_metadata()
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/pbr/version.py", line 478, in _get_version_from_importlib_metadata
    return SemanticVersion.from_pip_string(result_string)
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/pbr/version.py", line 151, in from_pip_string
    return klass._from_pip_string_unsafe(version_string)
  File "/var/lib/juju/agents/unit-cloudsupport-0/charm/venv/pbr/version.py", line 158, in _from_pip_string_unsafe
    version_string = version_string.lstrip('vV')
AttributeError: 'NoneType' object has no attribute 'lstrip'

I couldn't find any workaround for this issue yet. Also, the charm can't be downgraded to v4 because of the following issue:

$ juju upgrade-charm --switch ch:cloudsupport-4 cloudsupport --channel latest/edge
Added charm-hub charm "cloudsupport", revision 4 in channel edge, to the model
ERROR cannot downgrade from v2 charm format to v1