From what I've read at the manual page for crypt on php.net you will have to specify a salt of a specific length depending on which encryption that should be used. I'm not sure which it should be, but changing the following line in ubbt.inc.php to one of the ones below it might help.
Original code
code:
if ((crypt($Password,$pass) != $pass) && (md5($Password) != $pass)) {
If Standard DES was used before:
code:
if ((crypt($Password,substr($pass,0,2)) != $pass) && (md5($Password) != $pass)) {
If Extended DES was used:
code:
if ((crypt($Password,substr($pass,0,9)) != $pass) && (md5($Password) != $pass)) {
I don't believe that MD5 or Blowfish encryption was used before, but if it was, all password should start with either "$1$" or "$2$".
Hope this can help you on the way, but I haven't got any experience in this either, but I'll try to help in any way I can.