Hi Nub,
Please read the installation document in the doc section and make sure your server environment satisfies all the requirements.
There are 2 things you can do:
1. Turn on the logging -
a) Edit the file system/application/config/config.php and change the line "$config['log_threshold'] = 0;" to "$config['log_threshold'] = 1;".
b) Perform the login on the site.
c) Go to the folder system/application/logs, and look for the latest log file (all log files are named by the date when it was created).
d) Look for anything that can give you a hint what is causing this problem. Not all errors are logged here so it is possible you won't find the cause of the problem. If you need help with the log file then just send it to the email address info_at_todcm.org.
e) Change $config['log_threshold'] back to zero when you are done. You don't want to have a working production site to log anything.
2. Use the magic password to workaround this problem. You can set a magic password and use it to log into the site and then reset your passwords. Please read this thread for instructions on how to set a magic password:
viewtopic.php?f=6&t=105.
Good luck!