...

IF @vchtstsaddt IS NULL

BEGIN

SET @vchtstsaddt = NULL

END

Else

SET @vchtstsaddt = CONVERT(datetime, @vchtstsaddt)

...

This makes little or no sense. If the variable already is null, why set it to null?

On the other hand, each of the variables is declared as a nvarchar(30) data type. Most likely, one of those does not actually contain a value that can be converted to a datetime value. I would check the actual contents of those values to see if you have bogus values. In any event, you're then assigning the datetime converted value back to the nvarchar variable. What is then done with these values?