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Is there any further information with the error? Like the query itself that failed? I'm not seeing any issues in my tests with admin only lists. Seams like something was added to the query resulting in and invalid query. That's typically due to a bad filter or sorting rule.

but they still do not show up on the list!

That's a crazy old bug I completely forgot about.

forge.joomlapolis.com/issues/6467

They'll only display if you're viewing the userlist as a CB Moderator. Ensure you're viewing the userlist while logged in with moderator permissions. There wasn't a high priority to fix this since displaying blocked users to other regular users was largely pointless.

What are you trying to do with this userlist exactly?

Question 1: If I set all members through CB User Management to Enabled, this should have them show up in lists, but they will still need a membership to access the website.

If you've CBSubs configured that way sure that's how it'll work. I'm not sure what the question is here though.

Question 2: I think that when I imported the members, I Disabled all expired members, which may be the source of the issue. Would anything change them to Disabled later?

Having disabled users is perfectly fine and I doubt users being disabled is the cause of your problem. Your problem seams to be the query is breaking.