Grav wants to open _what_?!

I'm trying to run grav with lighttpd & php 5.6.20 on Windows 10. I have all of this under a restricted user (Lighttpd), and when I browse to the main page (/blog/index.php), I get the following error:

Unknown: open(C:\WINDOWS\sess_8vl4gv2cvnb27duoep0ihriaq4, O_RDWR) failed: Permission denied (13)

I'm at a loss for how to handle this. I assume it's due to a misplaced root for configuration of something, but whether that's lighttpd, php, or grav, I have no idea.

Could be a problem of the session.save_path directive in the php.inifile. It's trying to save a session to the C:\WINDOWS folder which I guess it's not the right one.

No direct experience with such an issue. Google is your friend. Or use MAMP on Windows, which is known to work fine.