Comment 6 for bug 1821415

Lubuntu is suffering what seems to be a related failure although I tracked that down to what I thought was a different upstream bug report. In Lubuntu's case the lxqt-policykit-agent suffers a SIGSEGV as a side-affect (I think) of polkit failing to authorise.

It was originally discovered when, using the GUI, an additional user was added with 'sudo' membership. This broke the subsequent ability of the new user to make any user account changes via the GUI because they'd get an authentication challenge and then failure. That failure seems to exercise a rarely followed code path that causes the SIGSEGV, and is likely unrelated to the underlying policykit issue.
That new 'sudo' member is also prevented from using privileged tools like "gparted".

bug #1828663 "policykit failures due to internal user id mismatch"