openstack-upgrade action fails on leader with keystone-ldap ("domain-backend") subordinate

This was observed with the 21.04 release of charm-keystone, during a Train to Ussuri upgrade, and I think also a Stein to Train upgrade (although I didn't recognize it at the time).

The environment in question has a keystone-ldap subordinate, and is thus using the domain-backend relation. The error was encountered when using action-managed-upgrades=true, after pausing hacluster and the keystone leader unit, then trying to run the openstack-upgrade action.

I've reviewed the code and am fairly certain this isn't immediately blocking me since there's been no change in domain name and the keystone-ldap-provided domain already exists, plus service restarts are implicitly being taken care of via the pause/resume on each of the units, so at least in my case, I think this error is benign - but nonetheless, it is alarming and might be more important on other environments; I'm not sure.

Tracebacks are attached.

Best Regards,
Paul Goins