One of the fields in the archive tables of version 3.3.03 is set to text, it is supposed to be memo.
If you havn't added any mods to your forum then upgrading to 3.3.04 or 3.3.05 will fix that for ya. Else you can go into the database with access and do it manually, it the X_MESSAGE field in one of the FORUM_A_XXX tables.
to expand a little on what Nathan was saying, once you find the correct field, you need to change the type from Text to Memo, and that should fix the error (if it is the same error I was getting when I tried to archive posts).