Hello,
I'm searching and reading for a while, but it seems there is no solution for my problem.
I'm running SilverStripe on Microsoft Webmatrix and I had to change my admin password through the database.
After that an error appears:

[User Error] Uncaught PasswordEncryptor_NotFoundException: No implementation found for ""
POST /Security/LoginForm
Line 59 in C:\Documents\My Web Sites\Security FAQ\framework\security\PasswordEncryptor.php
Source

50 	
51 	/**
52 	 * @param String $algorithm
53 	 * @return PasswordEncryptor
54 	 * @throws PasswordEncryptor_NotFoundException
55 	 */
56 	public static function create_for_algorithm($algorithm) {
57 		$encryptors = self::get_encryptors();
58 		if(!isset($encryptors[$algorithm])) {
59 			throw new PasswordEncryptor_NotFoundException(
60 				sprintf('No implementation found for "%s"', $algorithm)
61 			);
62 		}
63 		
64 		$class=key($encryptors[$algorithm]);
65 		if(!class_exists($class)) {

Trace
PasswordEncryptor::create_for_algorithm()
Member.php:194
Member->checkPassword(<filtered>)
MemberAuthenticator.php:52
MemberAuthenticator::authenticate(Array,MemberLoginForm)
call_user_func_array(Array,Array)
MemberLoginForm.php:237
MemberLoginForm->performLogin(Array)
MemberLoginForm.php:128
MemberLoginForm->dologin(Array,MemberLoginForm,SS_HTTPRequest)
Form.php:363
Form->httpSubmission(SS_HTTPRequest)
RequestHandler.php:184
RequestHandler->handleRequest(SS_HTTPRequest,DataModel)
RequestHandler.php:204
RequestHandler->handleRequest(SS_HTTPRequest,DataModel)
Controller.php:153
Controller->handleRequest(SS_HTTPRequest,DataModel)
Director.php:307
Director::handleRequest(SS_HTTPRequest,Session,DataModel)
Director.php:119
Director::direct(Security/LoginForm,DataModel)
main.php:126

I hope you can help me and sorry for my bad English btw, I'm from Germany.

Greetings,
Leon.

I had to change my admin password through the database.

Uhm, you do understand why this is a futile attempt?

To get you back in the CMS, add this to mysite/_config.php:

Security::setDefaultAdmin('123', '234');

Then go to 'mysite.com/admin', login with 123/234 and then give your administrative user a new password inside the CMS. After that you should remove the line from your config.

Okay, I see... SilverStripe is really different from the CMS I use usually.

I now tried to add Security::setDefaultAdmin('123', '234'); to my _config.php but I'm getting the same error still.

Did you tried to log-in with username: '123' and password: '234'?

Of course, I understand the method, but now SilverStripe says:
"That doesn't seem to be the right e-mail address or password. Please try again."