Thanks, that helped. I have found and deleted the respective misbehaving menu.
For a larger problem with YYYY-MM-DD-HH-MM type URLs, I went into phpmyadmin and table joomla_menu, columns alias and link, and bulk edited all those entries. They get generated as a last resort for an otherwise empty alias when UTF-8 titles are used on menus and saved (and I was too lazy to explicitly set them at the time). As the web server supports UTF-8 (including Czech + Russian Cyrillic, Arabic, Chinese) in the URL paths, I more or less simply copied the title column through.
Waiting now for Google to rank the site a bit better ...