I just experienced the same problem.
The problem seemed to resolve itself once I closed all open files (models, assemblies, drawings) in SolidWORKS.
Try that...hopefully it will work for you as well.
I've came across this again with another customer and running task scheduler after hours, or doing a pack and go of the assembly first then running TS on this file set has worked. It seems to be a case that the files are being used somewhere while trying to run TS and this is preventing them by being opened and exported. So even when the customer is sure the files aren't being used it's more than likely someone else is currently using them.