Expiration date of cookies set by any version prior to Tetra-WebBBS 6.10 ends on 2010-12-31, more precise in the first second of 2010-12-31. As a result browsers will delete those old cookies or at least won't send 'em any more to the server. Depending on how exactly a browser handles expired cookies, all cookie based features of older versions of the forum software will stop working on 2010-12-31. Effected is not only the "new" flag on message index pages but as well the information used to prefill posting forms, esp. the posters name and password.
While newer versions of Tetra-WebBBS provide a way for users to reset their profile password, older WebBBS 5.1x and 5.2x do NOT provide such a feature. It will become a nightmare for admins to process all "forgot my password" requests manually.
Solution to all this is pretty simple:
Upgrade your old forums to the current version of Tetra-WebBBS now. A bit more than 2 months left.