"Your account is authorized for the Wrath of the Lich King expansion[...]

So, I've been playing on Onyxia quite regularly, using the 3.3.5 client (the official original; not the "new models" mod).

Out of curiosity, I tried switching from Onyxia to Icecrown via the in-game realm selection, without closing the game (I've played on both Lordaeron as well as Icecrown in the past, using the same exact client, and the same exact account).

Upon doing so, however, I was given the following error:
"Your account is authorized for the Wrath of the Lich King expansion, but the computer you are playing on does not contain Wrath of thr Lich King data. To play on this machine with this account, you must install the Wrath of the Lich King. Additional data is avsilable at:[www.worldofwarcraft.com/lichking/download/]"

Upon closing the game and re-opening, I receive the same error when I attemmpt even logging in, thus, I cannot access any of the Warmane realms that support the official 3.3.5 client.

No new addons, nor any additional software was installed. I sinply attempted to change realms via the in-game menus, and now I'm seemingly locked out.

Has anyone elsr experienced this? If so, what's the solution, aside from completely re-installing?

Thanks very much in advance!

Have you tried...

1) Closing the client and removing the cache folder.
2) Closing the client and renaming the WTF folder. (It that doesn't work then you can rename it back)

Deleting the WTF folder and letting the game build a new one did the trick. Thanks!