I was getting the dreaded editor won't start at all error, and I wasn't looking forward to a complete re-install. So for fun I went into the registry and deleted the sandbox entries, then started the editor, and it worked fine, no more fatal error. I don't know exactly what registry entry was keeping it from starting, but it works ok now. you could try that.
I deleted the entire key - HKEY_CURRENT_USER\Software\Crytek\Sandbox The editor will recreate all the entries the next time you start it.