Weird error out of the blue. We have workflows in several libraries that will take a document and upload to a single library, it does so by changing some document properties, committing change then starting another workflow that will actually move the document. This has been working for months... now all of a sudden our users get the error "Failed to start workflow. Attempted to perform an unauthorized operation." I'm the site collection administrator, I can run the same WF with no issues. So it is not an APP Pool issue... No settings on Central Admin were changed and no permission sets that I'm aware have changed.
We just performed an IIS Reset... no change.
Well you can check to see if its a permissions issue, try the following: place your Start workflow action inside of an Action Set, under the Common tab of the Action Set configuration check the box "Run as workflow owner" and then have your user attempt to run the workflow. This may act as a work around in your situation, all else being a mystery as you said no changes have taken place.