I downloaded Ventura and can no longer fast-switch between accounts

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

Gordon Noel1 wrote:

When I downloaded Ventura, I was no longer able to fast-switch between accounts, or get my accounts to sleep: to move between accounts, I have to log-out to get to the other account's login page. that means dozens of times a day I have to reboot my computer. When I reboot, my name and fast switching appears in the menu bar, I can fast-switch once, but then neither account can fast switch again, and the user name disappears from the menu bar. So I have to log out both accounts and reboot.

The various on-line suggestions haven't worked.

What suggestion have you tried so far— or do we risk turning this into a game of Whack-O-Mole...(?)

Again post the entire report with the additional text button so we can peruse it and try to see if there's any software in there that might be contribution to the problem. We look at more than what you've provided.

I don't see any of the usuals suspects. There have been a number of issues reported that Dropbox might be involved with. You might try uninstalling it according to the developer;s instructions temporarily and see if there's any improvement.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into the Recovery volume.