I’d try the parts of safe mode individually:
- Plugins:
/?safe_mode=no_pluginsdoes it break? - Themes:
/?safe_mode=no_themesdoes it break? - Unofficial Plugins:
/?safe_mode=no_unofficial_pluginsdoes 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.