I’d try the parts of safe mode individually:
- Plugins:
/?safe_mode=no_plugins
does it break? - Themes:
/?safe_mode=no_themes
does it break? - 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.