copying from imap server to local folders fails always - local folders become empty or corrupt
For the last 2 days, I am trying to copy all my emails from 2020 (about 15K emails) from my IMAP server to a local folder (called '2020'). I was waiting for all the emails to be downloaded patiently until the end (i am not interrupting the process). After completing the copy, I am happy to see that the emails are transferred to the local folder. They all apppear nicely. Then when I restart Thunderbird, guess what: the folder appears empty, thunderbird attempts to creates the summary file, and after this, the folder appears completely empty... I tried this already 3 times. Note that this is not due to disk space (i have a few TBs left), or any disk errors. Repairing the folder does not work. In my profile folder, I see the local file called '2020' (no extension, about 1GB). The msf file appears to be 2KB (tiny, usually these were a few MBs). Deleting the msf file does not help either.
Opaite Mbohovái (1)
I would do this is smaller batches. Various things may throttle or interrupt the completion of the process on such a large number of messages all going at once. Gmail for example I believe restricts mass operations like this, you could exceed a limit from your data provider, and Thunderbird also needs time to index and organize after the download appears done. Start with say 2,000 messages, see if that is successful, work up to bigger batches (or down if it still fails).