in auth.c function auth_update
find

void auth_update( void )
{
   CHAR_DATA *vch;
   AUTH_DATA *auth;
   int count = 0;
   bool pfile, authsave = false;

   for( auth = first_auth; auth; auth = auth->next )
   {

change it to

void auth_update( void )
{
   CHAR_DATA *vch;
   AUTH_DATA *auth, *auth_next;
   int count = 0;
   bool pfile, authsave = false;

   for( auth = first_auth; auth; auth = auth_next )
   {
      auth_next = auth->next;

If you have a character waiting in the auth list and hes destroyed etc... when it updates the auth list it will crash.