E2120 Malformed Id

This error occurs when Salesforce expects a valid 18-character or 15-character ID format but receives an incorrect or improperly formatted value.

E2120 - ECE_SForceUpdateInsert: We got an error back from SalesForce when trying to update or insert data in SalesForce.
Error returned from SFDC on upsert: [fields: Invoice__c , message: Invoice/Credit Memo: id value of incorrect type: NOT LINKED TO INVOICE , statusCode: MALFORMED_ID ] , Record does not exist in SFDC, Data sent to SFDC: sObjects: [type: Sales_Summary__c , Unique_Key__c: USA#999 , Name: 000678 , Customer__c: 001f400000CyR2FAAV , Posting_Date__c: 2018-10-01T00:00:00.000Z , Document_Type__c: Invoice , Document_No__c: 91E688D36-B , Description__c: Rev adj. , Currency_Code__c: USD , Amount__c: 3.800000 , Salesperson_Name__c: John Doe , Open__c: false , Due_Date__c: 2018-11-15T00:00:00.000Z , Sales_LCY__c: 3.800000 , Date__c: 2018-10-01T00:00:00.000Z , Payment_Discount_Date__c: 2018-10-01T00:00:00.000Z , Invoice__c: NOT LINKED TO INVOICE.

Common causes: 

• Providing an incorrectly formatted Salesforce ID (e.g., missing characters).
• Using a non-existent ID in a query or DML operation.
• Assigning a non-ID value to a lookup or reference field.

Solution:

• Check this particular record in your source system and correct it by using a proper Salesforce Id. As soon as you correct it, the data should be transferred correctly. If you need time to check this records, you can filter it out on the transfer in question in the meantime. When you add a filter or make any change on the transfer, you always need to click on Activate Changes at the top of your page. 
• If the error message is showing a correct Salesforce ID but it fails, you would need to check in Salesforce if it still exists.