This is a quick and easy one. I came across a virtual appliance the other day that had an expired certificate.
When you click Next you’ll get an error saying the package is signed with an invalid certificate.
ovftool.exe --skipManifestCheck c:\tmp\old.ova c:\tmp\new.ova
You’ll then be able to deploy the appliance without it barfing. Remember, though, that this is a bit of a rough workaround, and you should really contact the appliance vendor in the first instance as they’ll likely be keen to fix the issue. In my case I was able to continue with my testing while the vendor went ahead and fixed things on their side.