You could delete your cookie file entirely. This will force you to reset all the preferences and info that those sites usually remember for you. But you’ll only have to do it once, and it’ll get rid of the “couldn’t understand” message.
On second thought, just rename the cookie file first, so that you can name it back if something even more disasterous happens.
The other possibility is that your bookmarks are pointing too deep into their website. On some sites, cookie creation might only take place on the first page. If you are bypassing that page and no longer have your original cookie, that might cause problems.
Actually, regularly emptying your cache will significantly slow down your internet performance. The purpose of the cache is to store graphics and pages so you don’t have to pull the data off of the internet every time. Your computer will say, “hey I already have that picture” and will pop it up in a split second. (just like when you hit the back button) The only way the cache could slow down your performance is if it fills your hard drive to the point where you don’t have enough space to properly use virtual RAM. Just limit your cache size to 1% of the drive size and you will be fine. View --> Internet Options --> Settings button in the middle on IE, I don’t know about Netscrape.