A. When App-V hits a problem with a malformed OSD file, it tracks the bad OSD in the registry at HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Softgrid\4.5\Client\IgnoredApps. So if you've resolved the malformed OSD, you need to delete the string value associated with the malformed OSD entry so the application will be able to launch.
Note that if you're using a 64-bit version of Windows, the registry key is HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\SoftGrid\4.5\Client\IgnoredApps (note the addition of Wow6432Node).
You can see in my example below that I had a bad entry for Adobe Reader. Also note it shows the time stamp of the cached OSD file.