I have recieved above error on mulesoft system, when trying to do upsert case object in salesforce.
According to the doc.
Number of synchronous concurrent transactions for long-running transactions that last longer than 5 seconds for each org: 10
It would seem like your mulesoft process, together with any other integration you might be having going on, goes over this limit. (so that you have more than 10 concurrent transactions that take longer than 5 seconds to commit)
Try reducing the amount calls / threads you call SF with and revise your code to try to get the transactions as fast as possible so they don't go over the 5 seconds and hence, don't count against this limit.