This issue occurs when the name of the deleted folder has changed on the server. Aqua Mail is still looking for the old name of the folder and cannot perform the action of deleting the messages.

To fix this issue go to Settings > Manage accounts > Select your account > Folders > 
Tap your Trash folder and select "Sync with Deleted" (this may already be the chosen option, re-select it if so) > Refresh the account.