Try version 91.3.0 from https://www.thunderbird.net/en-US/thunderbird/all/

Wayne, Thank you. That did not solve the problem. If I knew how, I would delete all of the data associated with Thunderbird after deleting the app. Then downloading the newer version might work.

Tried Help > Troubleshoot mode?

Wayne, Thank you. That did not solve the problem. I am wondering if it is a problem with gmail. I will try the same thing with Apple Mail app and gmail when I have the time.

The problem is not with gmail. I used the Apple Mail app and was able to save drafts. So it is either in the setting on Thunderbird or on my Mac. Interestingly, I have the problem on both my Mac and my MacBook. My wife does not have any problem saving drafts in Thunderbird on her Mac or MacBook

In Activity Manager, when saving a Draft or message to Sent folder, or moving from a local folder to IMAP:

"The current operation on 'Drafts' did not succeed. The mail server for account x@y.z responded: Expected literal, but saw UTF8"

Reproducible in any build of Thunderbird 91, including nightlies from this week, in Troubleshooting Mode, and even with a completely new profile. Happens on Windows and MacOS - across several OS releases of both.

It is not a transport (TLS/SSL) problem, as other IMAP operations work (moving, marking, flagging, deleting, etc)

It's not a (user) data problem either, as fresh accounts both client and server-side manifest this issue.

I am running ArchiveOpteryx as the mailserver, but nothing significant/useful is logged there either, as it's an MUA problem.

Been running Tbird/Mozilla/etc for decades - this is a new Tbird problem.

I've been having this problem since the 91.2.1 version. The current update didn't help. I now have 91.3.1.

Are you using AVG or any other AV software?

Wayne, Yes! Do you think that is the cause of the problem?

Wayne, Thank you. You solved the problem and it is AVG. I uninstalled AVG and restarted my Mac and Thunderbird worked properly, saving a draft and a template. I then reinstalled AVG, restated my Mac and Thunderbird would not save a draft nor a template.

Wayne, I thank you too. I also had AVG. Once I uninstalled it, Thunderbird is working like clockwork again. I appreciate you!

Wayne, I uninstalled AVG and then installed Avast (another free anti-virus). Thunderbird is working great. Thanks again.

I have the same problem - "Copy Failed". I also get "The current command did not succeed. The mail server for account "@mailerserver" responded: Expected literal but saw UTF8 (~(NNN)).". No antivirus running, just Windows antivirus. Previous version of T'Bird still runs OK. Problem reproduced on two Win10 machines. Only preference change is Security.tls.version.min set to 1 to maintain IMAP provider compatibility. Also applies to "Sent" emails saved to IMAP Sent folder.

Update 2021-12-09: Applied 9.1.40 (32 bit) and this problem still remains. Any updates?