This indicates that a call from your browser to Atlassian's media servers (where attachments are stored) timed out. If you're under paid support with draw.io, you can file a ticket via the links on https://marketplace.atlassian.com/apps/1210933/draw-io-diagrams-whiteboards?hosting=cloud&tab=support.
Otherwise, have a look at the network tab of the browser debugger to see what the error is.
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