Your mail provider has changed the name of the Drafts folder, and the app isn't able to upload the message there. Or maybe the Sent folder name.
- Long press on the account on the app's main screen
- Options and folders -> Folders
- Let it refresh
- Find the server Drafts folder, tap its name, choose "Sync as Drafts"
- Same with the Sent folder ("Sync as Sent")
- I'd check also Deleted and Spam and Archive (if you use the latter two)