For some very odd reason, it seems that 9 out
of 10 error messages like this deal with Firefox.
If "Your browser sent a request that this server
could not understand", plus "Size of a request
header field exceeds server limit", that strange
request may be a bad or unusually big cookie.
(The same thing is true for "Error 400" codes.)
Simply clear all of your cookies and everything
should be fine. Don't worry about getting rid of
them; absolutely nothing bad will happen. You
won’t lose your internet browser's "Bookmarks"
or "Favorites". The worst-case scenario is that
you will have to type your username/password
again when you first revisit YouTube and a few
other websites -- that's all.
While you're erasing your cookies, you should
also be deleting your "temporary internet files"
and "history" to get them out of your computer
memory, and make everything fresh. After that,
it's best if you shutdown and then reboot:
Internet Explorer 6:
→ Tools → Internet Options
→ Delete Cookies → OK
→ Delete Files → OK
→ Clear History → Yes
→ OK
Internet Explorer 7:
→ Tools → Internet Options → Delete...
→ Delete files → Yes
→ Delete cookies → OK
→ Delete history → OK
→ Close (browsing history window)
→ OK
Mozilla Firefox
→ Tools → Clear Private Data
[x] Browsing History
[x] Download History
[x] Saved Form and Search History
[x] Cache
[x] Cookies
→ Clear Private Data Now