This doesn't sound like a Thunderbird error.

When do you get this error - when you are first installing Thunderbird?

Every time I try to set up a particular email account, I get the same error after putting the account's password in: "can't access property 'cleanup, this [#configVerifier] is undefined"

I saw an answer elsewhere here that I had to 'white list' the Mozilla folder in my C: drive for CCleaner which I use. I did that, but the problem persists.

Thanks for any help!

John, Kit, what version are you using?

This might be https://bugzilla.mozilla.org/show_bug.cgi?id=1975814 in version 140.

140.0.1 I'm new to Thunderbird, and initially loaded the one on MSoft's app store. I was doing ok setting up my four different accounts, but a pop up suggested I use the 'Release' channel version, and that's when the trouble started! Can I revert to the 'nice and steady' first version, or must I re-install that one and start over?

Turn off "hub" in Settings to workaround.