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?