Previews for Search Results in Outlook Web App

User complains that, for one particular Exchange Online shared mailbox, previews will not load for search results. In Preview pane, it states "Unable to connect, check network..."

It is web app only, problem doesn't occur in client.

It is this user only, problem doesn't occur for other users of this mailbox.

Occurs on multiple machines and browsers.

Only occurs for search results.

Message age doesn't matter.