Thanks pbaldy, you were right! Though I am now completely embarrassed, I had them backwards. For all interested, the code would be:
DoCmd.OpenForm "DAS_Edit", acNormal, ,"[DAS_Edit] = " & Me.[Searchbox]
Not:
DoCmd.OpenForm "DAS_Edit", acNormal, ,"[Searchbox] = " & Me.[DAS_Edit]