Website has disappeared!

[panski]

Hello everyone.

I've performed a core upgrade from 1.2.4 to 1.2.6 after which the website completely fails to load.
I'm still able to access admin panel, however.
Error log is clean, so nothing can be read from there.
The only thing is, when I click on install extension - this error appears:
Fatal error: Call to undefined method ABackup::validate() in /home/mantisg3/public_html/mantis/core/lib/package_manager.php on line 805

I've manually replace package_manager.php file from Github but it didn't resolve the issue.
Please tell me how to resolve this problem.

[Basara]

Hello.

You should update 1.2.4 to the 1.2.5 and then to 1.2.6
If you can access Admin maybe problem with SF so try to disable .htacess file in your website root