Assuming crmfields is required, the following result was found.
= List:String(); l_MyMultiPicklist.add("Option1"); l_MyMultiPicklist.add("Options 2, 3"); // Push to Creator: m_CrmFields = Map(); m_CrmFields.put("My_MultiPicklist": l_MyMultiPicklist.toString(",")); // Yields ERROR: Failure, Invalid column value...