Shannon - 

Have you tried adding a pause into the workflow? Sometimes that will help "save pending changes" better than the actual save pending changes action because it will complete everything up to the pause. I have seen workflows process things to fast and it could get getting a head of its self and trying to update something that hasn't been set it.

Thanks for the clarification. It seemed like from the post you linked to the issue was with the update of an item and not with the flexi-task its self. I personally have found more success adding a small pause in the workflow in place of the save pending changes, so that is what I was suggesting to get the full commit done on the update item. How do you have your flexi-task set up. Does 1st response complete the task, all need to complete, or majority rules? Also, are you using lazy approval? Also, are you using the flexi-task as a means to get an "approve" or "reject" type outcome? If you are just using it to do more of a my work is completed type of task, I would also remove an unused branched and just have one called work completed for example to simplify the task action. 

Are you storing the outcomes in variables as well. You can store the individual outcomes in one var and the overall outcome in another. by doing this and logging them might help troubleshoot your issue.

I have seen some intermittent issues if users respond to fast to the action. 

Shannon -

Without knowing the data types of your update you could have a data type mismatch (examples: string to int, or int to number, etc) that could be causing the error or something fluky could have happened to your action. One thing you could try is deleting your update action and re-adding it to the canvas and also start with one update like the title or something easy, then move on to others one at a time to see where things go wrong and stop working. I know it can be a pain, but could help single out the column that is causing issues.