Fatal error: Class CI_Session_files_driver contains 1 abstract method and must there

I'm also seeing this issue intermittently (restarting Apache resolves).

My setup is:
- Windows 7 Pro SP1 64-bit
- CI 3.1.5
- PHP 7.1.8 (also seen using 7.1.2, as well as 5.6)
- Apache 2.4.25 (stand-alone, not WAMP or XAMPP)
- MariaDB 10.1.4

Settings:
- opcache is disabled
- have seen using both file and database sessions driver

Super frustrating issue. At the moment, I'm locked into a Windows environment. Any ideas or suggestions? Thanks in advance.

little question. after deactivating the opcache did you restart the server? And you have the same error?
pls check the phpinfo, if the opcache is deactivate.

edit: did you use ci 3.1.5? And no changes in the system folder.