Option Explicit
Sub BMTest()
 Dim tmp As String
 tmp = "2021"
 WB "BM1", tmp
 tmp = "52"
 WB "BM2", tmp
 tmp = "10x"
 WB "BM3", tmp
 
 tmp = "2021"
 WB "BM4", tmp
 tmp = "52"
 WB "BM5", tmp
 tmp = "10x"
 WB "BM6", tmp
End Sub

Public Sub WB(ByVal BName As String, ByVal inhalt As String)
 If ActiveDocument.Bookmarks.Exists(BName) Then
 Dim r As Range
 Set r = ActiveDocument.Bookmarks(BName).Range
 r.Text = inhalt
 ActiveDocument.Bookmarks.Add BName, r
 Else
 Debug.Print "Bookmark not found: " & BName
 End If
End Sub

/* This works (note the space)*/
KTP.EG.[BM1] [BM2] [BM3]

/* This fails */
KTP.EG.[BM4][BM5][BM6]

KTP.EG.2021 52 10x
KTP.EG.10x