If you have tried to set the htaccess from the administrators screen and it is now erroring or even after you have trincated the admin and action recorder table then to correct it you need to edit one file and delete another.

Edit admin/.htaccess and remove the section

##### OSCOMMERCE ADMIN PROTECTION - BEGIN #####
AuthType Basic
AuthName "osCommerce Online Merchant Administration Tool"
AuthUserFile xxxxxx/admin/.htpasswd_oscommerce
Require valid-user
##### OSCOMMERCE ADMIN PROTECTION - END #####

Then delete the file admin/.htpasswd_oscommerce.

Now shutdown all your browsers and start them up again and try to log in.

I have found that always works but am