The Sent folder has been renamed on the server, not accessible under the name known to the app (which it saved when you were adding the account).
To fix:
- Long press the account (on the app's main window) -> Account options
- Tap Folders, let it refresh
- Find the server side Sent folder (may be Sent Items, Sent Messages, etc.)
- Tap Sent folder's name and choose "Sync with sent"
I'd also recommend checking Deleted and Drafts