Interesting... Thanks for the report!
Seems they missed a change in the database in phpbb 2.0.20...
[Sec] Changed the way we create "random" values for use as keys - chinchilla/Anthrax101
http://www.phpbb.com/support/documents.php?mode=changelog#2019I modified the database, give it a try now and let me know if its still broke. I'll check back later to see if it's OK.
edit: i just modified it again... I had to search the php code to see how big the column should be - it is supposed to be 16 characters but was only 6... looks like a typo in the db update script... yup it is for some versions and is completely missed for other versions...