TYPO3 Core

No. & It has been a cristal clear installation with core and dummy.

Same error.
Tried fresh installation of TYPO3 4.7.16 and 4.7.17.
When I open the backend example.org/typo3 the error message occurs. Clearing temp-Files does not change anything.
Update from a earlier version (4.5.2) to 4.7.17 works and backend works fine.

same here.

fresh install of 4.7.17 the error message occurrs wehen i try to login. (the /typo3 page)

Same problem with fresh 4.7.17 installation (core/dummy)

I tried to manually require the class where it is missing inserting:
require_once('../../typo3_src-4.7.17/typo3/sysext/saltedpasswords/classes/class.tx_saltedpasswords_div.php');

An then I got this error:

PHP Fatal error: Class 'tx_rsaauth_backendfactory' not found in (...)/typo3_src-4.7.17/typo3/sysext/rsaauth/hooks/class.tx_rsaauth_loginformhook.php on line 46

So if the classes are there maybe the autoloading is broken?

4.7.15 works without any problems

Back-End works, if you first install 4.7.15 fresh, and then update it to 4.7.17!