italic means you can not edit anything about the folder. It is read-only, including it's contents and sometime including it's sub folders. That may be because the server has it marked that way, or it could be because of the way it is named. fundamentally I would recommend against any attempt to move entire folders of mail. I would also recommend against trying to move more than a few hundred emails at a time and I would suggest you make sure there is no antivirus slowing things by trying to scan the emails or the encrypted connection as the process is prone to silent failure, from timeouts mostly, but any outside influence like scanning makes the process even less reliable than in inherently is.

IMAP is a mail protocol with a focus on syncing mail to a server and is not designed to move entire mail accounts as folk appear to be trying to do these days. Personally I have to ask folk that are trying to do this sort of thing if they have fully thought out their strategy and consulted with an expert in the field about their migration needs. One of those IT consultants that has qualifications in the area. Not that smart nephew that spends a lot of time online gaming and on social media. (essentially like my kids, useless on anything technical but they live online and on the socials.)

See https://support.mozilla.org/en-US/questions/1371846

Thanks, both of you, for your helpful responses. My conclusion is that the process of moving emails from Thunderbird to a server is rather fragile, and if possible is better avoided. In my case it is possible to avoid it, and so I'm going to turn back and restore what emails I can, from the server to my own computer. Following that, I'll clean up the residual mess at the server.

This does bring up another question, though: Is there some Thunderbird reference somewhere which explains things like the italicized folder names, and what it means to subscribe (or not) to a server folder?