Error 500 after installing website builder

I've HC8 running on win 2008 r2, today I've installed Joomla website builder for one of my domain, but after that I can't log on to it and show me error 500, and I dodn't know how can I fix that and make joomla available.
 

I have the same problem.
 
In my case is due to a new security implementation between mysql and php.
In the previous versions mysql stored the login passwords in a 16chars hash field. 
The newer versions uses a 41chars hash field and PHP would like it veeeery much.
 
What happened to me?
When I create a new db user using the DB Manager, the password is stored using the correct way.
When I use  Click & Install user's password is stored using the old way.
 
You can try to solve problem in 2 ways:
 
1. Try to change the db username password using the db manager. It should update the hash into mysql
2. Use a mysql manager (such as phpmyadmin) and
 
a) check if that password is in the wrong format
SELECT LENGTH(Password) FROM mysql.user WHERE User = 'your_username';
b) if the result is not 41 (double check with a user that works)
SET old_passwords = 0; UPDATE mysql.user SET Password = PASSWORD('your_existing_password') WHERE User = 'your_username' limit 1; FLUSH PRIVILEGES;