This error message can occur when the Software Hub is busy preparing programs you have used before on the same computer. You can usually work around the problem just by waiting for a few minutes and then refreshing the browser page.
More details
Software Hub works by downloading and virtualising applications on demand. If you log in to a computer you've used before, the Hub will try to anticipate your requirements by preparing application data in the background.
When you start Software Hub or try to open a program that requires it, a small validation program is launched. However, it takes its place in a queue behind any preparatory process that are already running. So if you try to open a program in Software Hub and see the 'validation failed' message, you will need to wait a few minutes for the other processes to complete. Once the authentication has taken place, you can refresh the browser page and launch the program you want to use.
This issue is expected to be resolved in the next version of the Software Hub.
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