[TYPO3-english] Fatal error: Class 'tx_rsaauth_backendfactory' not found :-(

[J. Bakshi (জ বকসী)]

Hello,

I am facing a strange issue. A typo3 4.6.4 site was running fine, even the front end is still functional.
But I can't log into the BE any more. It throws error as

`````````````
Fatal error: Class 'tx_rsaauth_backendfactory' not found
````````````

I have replace the source; cleared the cache ; but still the same error.

How can I fixed this ?