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I’d try the parts of safe mode individually:

  1. Plugins: /?safe_mode=no_plugins does it break?
  2. Themes: /?safe_mode=no_themes does it break?
  3. Unofficial Plugins: /?safe_mode=no_unofficial_plugins does it break?

If it breaks on themes and none of the other ones, it’s an unofficial plugin, conversely, if it breaks on unofficial Plugins and themes it’s an official plugin. Otherwise, it’s a theme bug.