Error about APPENDUID when sending

I upgraded to the new version 10 client on Windows. Each time I send a message, I get this message presented as an error:

[IMAP] OK [APPENDUID nnnnnn mmmmm] Completed

I then see two copies of the sent message in the sent folder.

Anyone with a suggestion for how to fix this? It this a server-side config issue or a bug in the new client?

I have the same problem!
Did you solve it or do you have any solution?

I still have this issue. No changes were made to the mail server (which I manage) and no other clients exhibit this issue, so I assume it’s just a bug. However, I do have the ability to change the mail server if I knew why I’m getting this error.

It seems that every email I send it getting sent to people twice, too. I think it’s related, but I cannot be sure.