Hi,
The access denied error isn't happening in the folders beneath the site, its happening in the Windows\Microsoft.NET...temp file folder. I'm pretty sure thats where it caches the generated thumbnails, similar to the way it caches the JIT compiled code for ASP.NET in general.
I would try recycle the app pool and see if that corrects it. Touching the Web.config file should recycle the app.
Hope it helps,
Joe