Unique seems to be notUnique

[Karl Grob]

In the enclosed Database my problem is located at

Field UniqueKey in Table Referenztext

When importing (V4RB) the fields, the Field in RecID 158 is  
considered as not Unique. Importing without the Unique-Flag gives me  
the enclosed Database. If I try to set the Unique-Flag in VStudio I  
get a error too. So the problem seems to be within the Database- 
Engine. I just cannot see the reason why RecId 158 should be  
notUnique, but perhaps I am blind.
Does anybody see a solution?

(BTW: I need these Fields for Import-Purposes.)

Karl Grob