Fast user switching broken after 10.14.6 update

Previous to this update, I could have as many concurrent logins as I wanted, switching easily between them using the menu available at the top-right of my screen. I could simply click the user I wanted to log-in as and it would display the login screen for that user with a password prompt. After entering the password, it would quickly switch to that account without affecting the former account, leaving everything open and as-is. Nothing would be lost or closed out in any of the logged-in accounts between the switching. Since this update, however, I am only able to log in as one user. When I click on a user from the menu, the login screen with password prompt is displayed as usual, but once I enter the password, something goes wrong and one of two things happen: 1) I am logged out and the main login screen is displayed with all of the accounts, at which point I then have to select the account I want to switch to again, and enter the password again, before being logged-in to that account. 2) The "spinner" animation that is shown in place of the password prompt gets stuck, and I have to power off my iMac and then turn it back on. Obviously, power cycling results in total loss, but in both cases, I end up losing work. Even in the first case, it's essentially the same as logging out of the first user. All of my windows are closed, all my browser tabs are gone, and any unsaved work is lost. No fast user switching means loss of efficiency and continuity, and it is crippling to my work day. My productivity and performance are seriously suffering as a result. Where do I start? What can I do? Please help!