(EDIT: you got a post in before the below response. It may still help, though)
Clear CloudFlare’s cache completely.
While in Development Mode, make sure you’re trying to login with a cleared browser cache.
So long as the Page Rule you’ve set up for your login URL is set up correctly CloudFlare would not have any effect on it. So, make sure you have the exact URL for the login page in the Page Rule (no “*”).
If after doing this you still can’t login:
1.) Enter Development Mode,
2.) temporarily delete the Cache Everything directive,
3.) clear your browser cache, and again try to login.
If you can login at this point, it is proof positive that the Page Rule for the login URL is somehow not set up correctly. CloudFlare’s Page Rules interface is buggy and sometimes doesn’t actually change the order of rules when you drag and drop them; so be sure to verify the order is correct (after inputting the exact login page URL) by refreshing the page.
Note: If W3TC was not previously caching the login page it would not start automatically doing so, so I suspect the Cache Everything directive is somehow being applied to the login page, which leads me to lean towards the Page Rule for the login page not being set up properly (EDIT: either this, or your security is blocking login attempts)
AJ
P.s. As an aside, you really don’t need ithemes security while using CloudFlare. In fact, it is usually counterproductive.