Sometimes without reason - Class CI_Session_files_driver

PHP Code:

A PHP Error was encountered
Severity: Error
Message: Class CI_Session_files_driver contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (SessionHandlerInterface::open)
Filename: drivers/Session_files_driver.php
Line Number: 49
Backtrace: 


Sometimes without any reason I got this message and can't really do nothing.
Refresh doesn't help.
For me helps restart apache in xampp.

I didn't even touched session driver or anything in system folder.

I think it is bug.