After installing mybb 1.8 in my local computer i was unable to login, i guess the cache is not working.
And I guess this is one of the worst bug reports I've ever read...
seriously:
1) New bug is a really bad description for a bug
2) Why is the login not working - what happens when you try to login?
3) Why are you assuming a cache problem?
I cannot reproduce your issue with the latest code from github...
Also, make sure to clean old cookies in browser if you installed MyBB multiple times on localhost.
if you installed MyBB multiple times on localhost or same domain/sub domain
you have to delete browser cache to fix login problem.