Unable to Delete Messages
I am new user with Vivaldi. When I try to delete messages, a pop-up indicates: "An Error Occurred. Server Error. UID Move. Mailbox Doesn't Exist: Trash"
Sorry, is this about Vivaldi Browser (which version) or Vivaldi.net email (or both)?
Also, which platform are you on?
On Windows 10 with Vivaldi Browser Snapshot 1.1.453.30 I am able to delete messages from Vivaldi.net email server.
First, create a folder named Trash not TRASH.
Delete your message in the inbox. Then go to Trash folder and delete it one more time.