Whether the 'Update Item' action is working fine for the same case. Can you check it.

Unfortunately since it's an update to an item on a list in another site, the 'Update Item' action isn't feasible.

do you have sufficient permissions on target list and items to be updated?

don't you have any validations on list or list field that may reject the update?

aren't there mandatory fields on items to be updated without a value yet (eg. Title)?

Are you getting the data from "Value"? Have you tried using "List Lookup"?  Also, is the field you are trying to update Managed Metadata?

I've tried to get the data from both 'Value' & 'List Lookup'.  The field I'm updating is just a simple multi-line text field.

Will double check all of these & be back in touch next week. I appreciate the questions!

Ok, so here's the answers after double checking.  None of these immediately appear to be the cause, but still considering the mandatory field relationships.

Answers:

• I have full control of the target & source lists, w/ no item level permissions applied to either the target, nor the source list.
  
  
• There is no validation applied to any of the list fields (neither source nor target list0

• The mandatory fields on both lists are identical, & all are populated on the source & target list item. The only question about this is if "Update Multiple Items" affects fields not targeted in the workflow (i.e. if it possible erases the target's mandatory fields data if not specifically assigned for update in the action?)

Thanks again everyone,

Jason

P.S.  if noteworthy, the workflow is actually the same thing that creates the item on the target list w/o error (so, I'm doubly sure list & item permissions, validation & mandatory fields aren't an issue).  The flow is initiated by a boolean field on the form, and then it's a simple decision point in the flow to determine if it's been previously created or not.  If new, it creates it in the target, & if it exists in the target already, it runs the "Multi Item Update" action on the existing item (and fails).  That's the end of the workflow though.

can you write value of 'Accomplishment' to history log and check it really contains expected value?

make sure there are not leading or trailing spaces in the value.

how do you check whether workflow runs for new or existing item?

can not it happen workflow goes down the update branch when the item do not exist yet?

can you post configuration of 'Description' field?

I know it's three month ago, but I was facing the same issue today and I found out, that this error appears, when a field value (in case of List lookup) that should update a field in the list is empty. If all fields have values the workflow runs as expected. No clue why the behavior is like this.

I am having the same error, but I am converting my Word document to PDF, but then when it gets to updating multiple items, it fails and I get the error.  Strange thing is that it works on a different library, the file converts and publishes the pdf with the fields to another library and does not fail.  The workflows are exactly the same except for they are being used on different libraries.  I have 1 required field (Description) also.  

Would really like to have this working as soon as I can, any ideas?

I just did another test of my publishing workflow and it worked this time.  the only difference I can see is that the file existed in the library I'm copying to, so it overwrote it.  Could it be that since it already existed, it had all it's meta data fields filled in and there wasn't anything to update?