When you look at the audit history for the work object, did another user or agent process the item prior to your submit attempt?

Here is another post that appears to be relevant to you issue:

https://collaborate.pega.com/question/please-resubmit-message

Hello,

Please let me know if you are doing the below thing at your end :

You have created a case and then wait for the few sec interval for the SLA to run. During this process you have not closed the assignment ,When your SLA works on an assignment it first acquires a lock on the assignment, performs the activity ( e.g sending emails )  and then release the lock.
This definitely modifies few of the work object information , so after the SLA finished its work, when you want to submit assignment
At this point if you want to work on the already open assignment , PRPC will ask you to refresh the page in order to make sure that the recent data are available in the page.
So this is expected behavior.

You can check the link shared by Carissa.