Looks like you actually have 2 problems: 1) os-prober giving information in latin1 and not utf-8. Can you run: sudo os-prober > report.bin then send resulting report.bin? Don't forget .bin to avoid file to be reencoded 2) grub fails on incorrect UTF-8. I fixed it in upstream but it's not packaged yet. Can you try latest upstream?
-- Regards Vladimir 'φ-coder/phcoder' Serbinenko
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