Hey There
How is the Workflow for the Form Item triggered? Is it set to run just when anything is changed, or does it also have a condition to say when a field is set to ......... etc
From my first thoughts I am thinking that maybe you need to add a condition into the workflow meaning that the user can press the save button, without triggering the workflow fisrt time.
I did a project recently and incorporated 2 different rules within the form. one save button just save the form to the database (form library) and the other button did the same, but added a value to a hidden field /variable.
I then set the workflow to start only when when this variable had the value allowing the user to save the form as many times as they wished using the first button and only firing the workflow when they hit the second button.
In regards to coming up with a way of using 2 buttons, there are a few options. In my case, the forst button was labelling "save and Carry on" and the second was "Save and continue later" as part of the workflowI added an action to send me a email with the link to the form, so suser could acrry on at a later date.
In your scenario it sounds like you have not placed a condition, so that workflow is firing off when anything changes to your file. Depending on what you have setup as the Workflow, unless it completes, it will state that someone else is using it. So you maybe want to try both the condition issue and also try to edit the file once/if your current workflow completes and see if you get the same result.
Hope that helps
Drop me a line if you have any questions
Ryan