I'm using Thunderbird 68.6.0 on Windows 10. I'm accessing an IMAP server from a well established email provider, which I have used for years. I have also used Thunderbird for well over a decade. I am attempting to use Thunderbird to backup my IMAP account locally. I installed Thunderbird from scratch on a separate machine. I connected with no problems. For some reason Thunderbird arbitrarily was subscribed to some folders but not others; I don't know why it was subscribed to some folders and not others on a fresh install. Nevertheless I right-clicked on the email account, selected "Subscribe...", refreshed the folders, selected all folders (hundreds of them), and clicked the "Subscribe" button, and then "OK". After a while Thunderbird started downloading messages. I let Thunderbird run overnight, but this morning perhaps half the folders have not been downloaded. I looked at the subscribed folders, and only about the first half of them are selected. I tried to select the second half, but Thunderbird apparently ignores the updated selection. That is, I will select some new folders after the "cutoff line" where folders are no longer subscribed, and I'll select "OK", but when I go back into the subscribed folders, the new folders I selected are no longer selected. It doesn't matter if I click the new folders manually, or if I select several folders and then hit the "Subscribe" button. It doesn't matter if I select a folder immediately after the "cutoff line", or if I go to the end and select the last few folders. Whatever I do, Thunderbird does not add new subscribe folders after a set number of selected folders. (I counted and it's way over 256; then I stopped counting.) Does Thunderbird have a limit on the number of folders it can subscribe to?