There was an error processing the row or the task was cancelled. No data was sent to Salesforce.

Cause

When you receive this error in the errors CSV file notice that most of the times this is caused by some custom code being triggered by the task (for example a Trigger) on your Salesforce. This code might be having some error or (usually) taking too much time to finish processing each batch.

If the performance of that process is poor, the bigger the batch is, the more time it will take to process it and this may lead to a query timeout on Salesforce side causing the job to be cancelled and this error being reported.

Workaround

A possible workaround (and can also be used to validate if this is the cause of your error) is to set the task to run using the Batch API and set the batch size to 1. With such a small batch, your trigger won't reach the timeout and therefore the task will finish successfully (unless the process is not just poorly performant but failing). This will cause the task to take longer to finish and therefore is not a valid solution in case you are processing many records.

Solution

The solution for this issue is to correct the error or improve the performance of the process being triggered. This needs to be done by a Salesforce Administrator and in case of doubts you should contact Salesforce support.