As I mentioned, I am not sure what cache it is referring to, but one is in the User\Local\Temp folder, which is set in Nero. You might try deleting something in there, although another user said that did not help.
I also know that most of the logs show references to Nero Caching files while burning, which is what I think it is referring to. I don't know exactly where this is, but it may just depend on available space on the hard drive. If there is no room to add 4.7 gigs worth of files prior to burning, you might get that message.
There is an option to cache or not cache files somewhere, I can't find it right now, but you might try, even though this is not recommended.
Under the UltraBuffer tab in options in NeroBurning ROM, there is another setting you might try for cache size.