Im not seeing this happening when Im trying to test, and the errors themselves would indicate the file simply isnt there when the guests are visiting. Thats likely a timing difference between the file actually being written, and being able to read it. Please update to the latest release in the first instance and test. If you are then still seeing the same problem, try switching over to mysql for data storage. You can do this from System->Advanced Configuration->Data Storage. if its an issue with disk writing, mysql may well serve you better.