The following issue (and similar error messages) are usually thrown if you're experiencing an issue with cache memory:
There are usually two quick and easy solutions to this issue. The first (and quickest) method is to open up your browser in incognito mode. Here's an article that shows you how to do that based on which browser you're using: https://balsamiq.com/support/faqs/privatebrowsing/
The second method is to clear your cache. Here's how to do that: https://balsamiq.com/support/faqs/clearcache/
If neither of those methods work, try restarting your device or reloading BOD on a different device and that might also resolve the error. Otherwise, please use the yellow feedback button in BOD to submit a tech support ticket and we'll get right on it!