Perhaps you should try some spelling variations:
"Grimm, Friedrich Melchior Freiherr von 1723-1807" » "Grimm, Friedrich Melchior Freiherr von 1720-1807", "Grimm, Friedrich Melchior Freiherr von 1743-1807", "Grimm, Friedrich Melchior Freiherr von 1783-1807"
"Grimm, Friedrich Melchior Freiherr von 1700-1807" » "Grimm, Friedrich Melchior Freiherr von 1707-1807", "Grimm, Friedrich Melchior Freiherr von 1708-1807", "Grimm, Friedrich Melchior Freiherr von 1744-1807"

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• Performing a fuzzy search may retrieve terms with similar spellings: (("Grimm, Friedrich Melchior Freiherr von 1723-1807") OR ((("Grimm, Friedrich Melchior Freiherr von 1700-1807") OR ("Grimm, Friedrich Melchior Freiherr von 1749-1807"))))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"Grimm, Friedrich Melchior Freiherr von 1723-1807\") OR (((\"Grimm, Friedrich Melchior Freiherr von 1700-1807\") OR (\"Grimm, Friedrich Melchior Freiherr von 1749-1807\"))))".
• Removing quotes may allow a broader search: (( Grimm, Friedrich Melchior Freiherr von 1723-1807 ) OR ((( Grimm, Friedrich Melchior Freiherr von 1700-1807 ) OR ( Grimm, Friedrich Melchior Freiherr von 1749-1807 )))).
• Adding a wildcard symbol may retrieve word variants: (("Grimm, Friedrich Melchior Freiherr von 1723-1807") OR ((("Grimm, Friedrich Melchior Freiherr von 1700-1807") OR ("Grimm, Friedrich Melchior Freiherr von 1749-1807"))))*.