Thunderbird: Copying emails unreliable

[Netherprovinc3]

Linux Mint 19.1 Cinnamon 64 bit
Thunderbird 78.8.1
with ImportExportToolsNG

I am copying emails from IMAP emails accounts to "Local Folders."
The software seems very buggy. Here are my experiences

1- Copying a folder by dragging the whole folder to become a subfolder of a folder in "Local Folders."
Result: Sometimes the copy doesn't happen and no error message is given.
Things that have fixed the problem: 1) copying the folder to be a subfolder of a different folder; 2) rebooting Thunderbird.

2- Folder copies but the contents in the folder does not copy.
I can't figure out how to fix this. I have tried 1) to drag emails there. No error message but nothing populates; 2) Right click -> move to -> . No error message, nothing populates.
Edit: I have been able to copy the emails one at a time for one of the folders. But, the most recent email did not copy (the one with the most recent date).

I am having a tough time believing my eyes. Sooo buggy?
The only thing that I can think to blame it on is ImportExportToolsNG.
Edit: I disabled ImportExportToolsNG and the problem seems to remain.

Edit: now Thunderbird occasionally gives an error message saying something like, "The operation failed because another process is using that folder."

[mikeflan]

1) You should not modify anything in the /.thunderbird/ directory unless Thunderbird has been shutdown/closed. It's generally a good idea to do File - Compact folders before shutting down Thunderbird if you plan to replace mail folder files, but this is not absolutely necessary.

2) You should only create mail folders from within Thunderbird (while it is running). Then after shutting down Thunderbird you can copy and replace the file with no extension that represents a TB folder (i.e. Inbox) if you want to, but you probably should save the old one for a little while in case it is needed later. The .mfs file does not need to be replaced, but can also be replaced. When all changes are made restart Thunderbird.

[Netherprovinc3]

I didn't modify anything outside of Thunderbird. At least not at any time that I can remember.

I think I have figured out the problem.
When one of the emails that I am trying to move isn't downloaded, this causes problems. So far, I have found 2 emails for which the content of the email is not downloaded. Only some of the email (the subject and who it was from) is downloaded.

I very much dislike Thunderbird now.
At the very least, it doesn't seem to produce proper error messages. Maybe it would be better if the software crashed instead of no error message. Or am I missing something and this is user error?

One positive thing is that copy helps you find emails with this nonsense.

The body of this message has not been downloaded from the server for reading offline. To read this message, you must reconnect to the network, choose Offline from the File menu and then uncheck Work Offline. In the future, you can select which messages or folders to read offline. To do this, choose Offline from the file menu and then select Download/Sync Now. You can adjust the Disk Space preference to prevent the downloading of large messages.

[mikeflan]

So far, I have found 2 emails for which the content of the email is not downloaded. Only some of the email (the subject and who it was from) is downloaded.

Thanks for the warning. I have never seen this happen in TB. I have seen something similar on messages slated to be sent in Office365 Outlook, but not sent yet.

Do you know what would cause a message not to download completely? Attachment too big? Internet too slow? interrupted by something else?

[Netherprovinc3]

So far, I have found 2 emails for which the content of the email is not downloaded. Only some of the email (the subject and who it was from) is downloaded.

Do you know what would cause a message not to download completely? Attachment too big? Internet too slow? interrupted by something else?

It has been about 1 year since the email account was online. So, my memory is a little hazy. The email account is not that big so I think I tried to click on every message to let it load. I suppose Thunderbird alleges that I didn't click on several of the messages. I'd say be careful with (1) downloading the content (the body) of emails; (2) downloading attachments.