YOU GOT THE DREADED "BAD REQUEST".
If "your browser sent a request that this server could not understand"
plus "Size of request header field exceeds server limit", that request
is a bad or oversized cookie. (The same thing is true for "Error 400").
For some very strange reason, it seems that
most Bad Request messages involve Firefox.
Simply delete your "Cookies" and everything
will be okay. Don't worry about erasing them;
nothing bad will happen. You won't lose your
internet browser's "Bookmarks" or "Favorites".
The worst thing that could happen is you will
be asked to retype your username/password
when you return to YouTube and some other
websites -- that's all.
Just as important, you should also delete all
your "Temporary Internet Files" and "History".
Those steps will make your computer totally
"fresh" in YouTube's eyes and make loading
and buffering faster. It would also be better if
you shutdown and reboot after doing this:
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
Internet Explorer 8:
→ Tools → Delete Browsing History
[. ] Preserve Favorites (leave blank)
[x] Temporary Internet files
[x] Cookies
[x] History
→ Delete
Mozilla Firefox 3:
→ Tools → Clear Private Data
[x] Browsing History
[x] Download History
[x] Saved Form and Search History
[x] Cache
[x] Cookies
→ Clear Private Data Now
Apple Safari 3:
→ Edit → Preferences
→ Security (tab) → Show Cookies
→ Remove All → Done
after that:
→ Safari → Empty Cache → Empty
after that:
→ History → Clear History
Google Chrome:
→ Tools (wrench) → Clear browsing data...
[x] Clear browsing history
[x] Clear download history
[x] Empty the cache
[x] Delete cookies
Clear data from this period: → Everything
→ Clear Browsing Data
DON'T FORGET TO SHUTDOWN AND REBOOT
AFTER DOING ONE OF THE ABOVE.