CryptPad

CryptPad is a collaborative office suite in which documents, spreadsheets, presentations, code snippets, and whiteboard drawings are encrypted in the browser before they are sent to the server. Pads support real-time multi-user editing with cursors, chat, and access management, and content can be given an expiry date by the person who created it.

Organisations use it for meeting notes, shared drafts, and whiteboard sessions where the content must stay out of server operator hands.

Running it yourself keeps that end-to-end encryption scheme under your control, and since storage is mostly plain files, backups and restores are straightforward.