According to the various sources there appear to have been a number of Dutch versions of plays by this name.

Het Geheim by Christian August Vulpius (1762 – 1827)

A Dutch translation of Das Geheimnis, a German play in 5 acts by Christian August Vulpius (1762 – 1827). Published in Leipzig in 1800.

Translated into English as The Mystery by M. Geisweiler c. 1800.

Kulturstiftung der Länder: Bibliothek der Deutschen Literatur - Bibliographie und Register, p. 423[1]

Performances in South Africa

1855: Performed as Het Geheim on Tuesday 12 June 1855 by Door Yver Bloeit de Kunst in the Garrison Theatre, and credited to "the author of Rinaldini" - i.e. Vulpius). Also performed were De Helleveeg (Loosjes) and Het Groot Ballet. The evening was in support of the "Patriotic Fund" for the Crimean War. The production was possibly repeated on 28 June.

Sources

http://en.wikipedia.org/wiki/Christian_August_Vulpius

F.C.L. Bosman, 1928: p462-3

Het Geheim by François Benoît Hoffman

A Dutch version of Le Secret, a French one-act opéra based on the comedy La Femme Jalouse by François-Antoine Jolly. The libretto by François Benoît Hoffman (1760–1828) and the music by Jean-Pierre Solié (1755- 1812). It was first performed by the Opéra-Comique in the Salle Favart on 20 April 1796. Published in 1796 as a "comédie mêlée de musique" ("a comedy with music").

Translated into Dutch as Het Geheim by Pieter Gerardus Witsen Geysbeek. Published by Abraham Mars (Amsterdam), 1801.

Sources

http://books.google.co.za/books/about/Het_geheim.html?id=holWAAAAcAAJ&redir_esc=y

Het Geheim by A de Bruine

The last production by Door Yver Bloeit de Kunst under the leadership of the 71 year old Johan Combrink, at the Good Hope Theatre 4 June, 1910. Boonzaier wrote a devastating criticism of what was apparently a ludicrous melodrama, badly presented, thus it was most probably another Dutch version of the Vulpius text.