Try:
SET @Dynamictbl = N'SELECT DISTINCT [Projektdefinition DB] AS Zugänge
FROM ' + @Table_Name + ' WHERE [Geschäftsbereich] = ''PB''' +
' EXCEPT
SELECT DISTINCT [Projektdefinition DB] AS Zugänge
FROM ' + @Table_Name2 + ' WHERE [Geschäftsbereich] = ''PB'''
or simple remove [
'
] before
Except
statement.