In our 2007 MOSS environment we have started seeing an intermittent problem with approval workflows being cancelled. This seems to be random and only started happening a few weeks ago. I cannot see anything relevant in the server ULS log files nor in the document history or workflow reports.
The error says that the workflow has been cancelled because the document was edited, which would be what I'd expect (the workflow is configured to cancel on document change) but the document has not been changed by anyone. There is no record of an edit in the document version history and the workflow fails after it has been approved by a number of people (usually around 4 but this depends).
My current working hypothesis is that a system process is changing the file silently but I do not know what would be doing this, perhaps search? but that seems like a long shot.
The workflow is an OOTB approval workflow tied to a simple document content type which has been configured to send the approval tasks in parallel rather than sequentially. This has been working without problems for more than a year.
Does anyone have any ideas what could be causing the problem because I am out of ideas?