Steps to reproduce the problem

1. Bookmark a (questionable) post P originated by user X on a remote instance.
2. Wait until or orchestrate so that the post P is deleted and your local moderators suspend user X.
3. From web client Settings > Import and export > Data export, try to export Bookmarks as CSV.

Expected behaviour

The bookmarks should be downloaded to the browser.

Actual behaviour

There is an error with text "We're sorry, but something went wrong on our end" and the oops.png image.

Detailed description

The problem arises when (1) a user has bookmarked a post on a remote instance, (2) the post is deleted (by the remote person who originated it and/or by the remote moderators), and (3) the user's home instance suspends the originating remote user (possibly also as well if the local moderators defederate completely from the remote instance).

After this, on the Mastodon web client, the local user is unable to download their bookmarks using CSV export. The error page described above and titled "This page is not correct - Mastodon" appears instead.

(It is still possible to get the bookmarks without the offending one as part of an ActivityPub-format exported data archive, but only once every 7 days.)

This may be related to pull request #18174 "Fix 500 error when a bookmark or favorite has been reported and deleted", but the occurrence of the error during attempted bookmark export must be in a different context from what happened for #18174, which is not described in detail there.

Specifications

Mastodon v4.0.2
Web client on Safari browser on iPad running iPadOS 15.7.2