Seguramente ciertos caracteres echando a perder tu sentencia SQL, te sugiero que utilices consultas parametrizadas mediante el objeto command.
Código:
Dim qry
Dim ObjConn
Dim rs
Dim param
Dim cmd
Set ObjConn = Server.CreateObject("ADODB.Connection")
Set cmd = Server.CreateObject("ADODB.Command")
'Siendo campo varchar de 300
caracteres = "--%'"
qry = "INSERT INTO tbl_algo(campo) VALUES(?); "
Set param = cmd.CreateParameter("nombre_parametro", adVarChar, adParamInput, 300, caracteres)
cmd.Parameters.Append(param)
ObjConn.Open strConnect
cmd.ActiveConnection = ObjConn
cmd.CommandType = adCmdText
cmd.CommandText = qry
cmd.Execute()
Set cmd = Nothing
Set param = Nothing
ObjConn.Close
Set ObjConn = Nothing
Saludos!